(CNN) -- Here's a pop quiz for anyone who calls the Rev. Martin Luther King Jr. an American hero.
Can you name any of his great speeches or written works without citing "I Have a Dream" or the "Letter from Birmingham Jail"?
Most Americans would likely flub this quiz. King may be a national hero whose birthday the country commemorates on Monday, but to many he remains a one-dimensional hero -- the vast body of his work unknown. Though he wrote five books and delivered up to 450 speeches a year, he's defined by one speech and one letter.
What then are the great works by King that never get the attention they deserve?
That's the question CNN put to some members of King's inner circle as well as top King scholars. We asked them to pick their favorite overlooked gems from King, any extraordinary spoken or written words people don't typically hear during King commemorations.
Six entries made our final cut: three sermons and speeches, King's most radical book, an astonishing letter he wrote as a college student, and a "eulogy" he delivered for a friend that revealed a side of him the public rarely saw.
'A Time to Break the Silence'
Sermon delivered at Riverside Church in New York on April 4, 1967.
Why it's important: This was King's most controversial speech. Even some members of his own staff warned him not to give it. With this sermon, King decisively came out against the Vietnam War at a time when many Americans still supported it. People were furious. President Lyndon Johnson stopped talking to him. Civil rights leaders criticized him, and major newspapers told him to stick to civil rights. Yet King put principle over personal popularity and continued to oppose the war. One year later to the day he gave this speech, King was assassinated in Memphis, Tennessee.
What he said: Money that should have been spent on Johnson's War on Poverty was being lost in Vietnam's killing fields. He said, "A nation that continues year after year to spend more money on military defense than on programs of social uplift is approaching spiritual death." The speech distilled King's belief that racism, economic exploitation and war were all connected as "triple evils."
Signature lines: "We are taking the black young men who had been crippled by our society and sending them 8,000 miles away to guarantee liberties in Southeast Asia which they had not found in Southwest Georgia and East Harlem. So we have been repeatedly faced with the cruel irony of watching Negro and white boys on TV screens as they kill and die together for a nation that has been unable to seat them together in the same schools."
What others say: "It's Dr. King's most underappreciated speech," says Vorris Nunley, a professor of rhetoric at the University of California, Riverside. "Former supporters, black as well as white, backed away from this too compassionate, too radical, too political King."
'Our God is Marching On!'
Speech delivered on March 25, 1965, in Montgomery, Alabama, at the end of the Selma-to-Montgomery march.
Why it's important: It is one of King's most electrifying speeches. When the roaring crowd joins King in shouting "Glory hallelujah!" at the end of the speech, the march becomes a church revival. The speech was the culmination of one of the movement's most brutal but critical campaigns. Three civil rights activists were killed and other marchers were beaten at the Edmund Pettus Bridge. King gave his defiant speech while standing on the steps of the Alabama State Capitol in Montgomery, a city known as the "Cradle of the Confederacy." This was the high-water mark of the civil rights movement. The Selma campaign would spark the passage of the 1965 Voting Rights Act.
What he said: King praised the white clergy and laypeople of various faiths who traveled to Selma to face danger with African-American protesters. He said that segregation was "on its deathbed" and the movement must now be prepared to "march on poverty."
Signature lines: "They told us we wouldn't get here. And there were those who said that we would get here only over their dead bodies, but all the world today knows that we are here and that we are standing before the forces of power in the state of Alabama saying, 'We ain't going to let nobody turn us around.'
"I come to say to you this afternoon, however difficult the moment, however frustrating the hour, it will not be long, because 'truth crushed to earth will rise again.' How long? Not long, because 'no lie can live forever.' ... How long? Not long, because the arc of the moral universe is long, but it bends toward justice."
What others say: The speech marked the triumphant end of the first phase of the civil rights movement -- seeking legal and political rights -- and the beginning of a new phase focused on economic inequality, says Jerald Podair, a history professor at Lawrence University in Appleton, Wisconsin.
"We see King at a moment of triumph," Podair says. "He is on the steps of the Alabama State Capitol, near the Dexter Avenue Baptist Church from which he launched the Montgomery bus boycott, as (Alabama) Gov. George Wallace cowers in his office with blinds drawn. But he also has miles to go. America is still an economically and socially divided nation.
'The American Dream'
Sermon delivered at Ebenezer Baptist Church in Atlanta on July 4, 1965.
Why it's important: We've heard about King's dream. But just two years later he told an audience that his dream had turned into a nightmare. King's sermon addresses questions that could have been snatched from today's headlines: What is a living wage for workers in menial jobs? Is income inequality as corrosive as racial injustice? What are the challenges of preserving a multicultural democracy?
What he said: King said that class divisions within the United States "can be as vicious and evil as a system based on racial injustice." King also talked about the dignity of all work, saying that even menial workers should make enough "so they can live and educate their children and buy a home and have the basic necessities of life."
Signature lines: "About two years ago now, I stood with many of you who stood there in person and all of you who were there in spirit before the Lincoln Memorial in Washington. As I came to the end of my speech there, I tried to tell the nation about a dream I had. I must confess to you this morning that since that sweltering August afternoon in 1963, my dream has often turned into a nightmare.
"I've seen my dream shattered as I've walked the streets of Chicago and see Negroes, young men and women, with a sense of utter hopelessness because they can't find any jobs. ... I've seen my dream shattered as I've been through Appalachia, and I've seen my white brothers along with Negroes living in poverty. And I'm concerned about white poverty as much as I'm concerned about Negro poverty."
What others say: "The dream is now clearly tied to equal job opportunities and decent wages," says Thomas Jackson, author of "From Civil Rights to Human Rights: Martin Luther King, Jr., and the Struggle for Economic Justice."
"King asks: How are you going to have a multiracial democracy if inequality makes life is so harsh and competitive at the bottom, where society is most multiracial and multinational?"
Delivered just before the 1963 campaign in Birmingham, Alabama.
Why it's important: "Every hero becomes a bore at last." That quote from the 19th century writer Ralph Waldo Emerson could have applied to King. With the rise of the Black Power movement, King seemed dull and obsolete to many youths in the late 1960s who preferred the fire of Malcolm X and the Black Panthers. King dressed like an undertaker and carefully measured each word in public. But King's "mock eulogy" reveals that in private King had a wicked sense of humor. He was a man who nicknamed one of his top aides, "Lil' Nigger," drank Harveys Bristol Cream sherry and smoked in private, and liked "playing the dozens," an African-American tradition of friends good-naturedly trading insults.
What he said: The moment is grim. King is holding a meeting of top staffers in 1963 just before they initiate a campaign in Birmingham. He warns them that some may not return alive. Then he breaks the tension by assuring them he would preach a marvelous tribute for anyone who doesn't make it. He proceeds by delivering an over-the-top eulogy for one of his aides at the meeting, Andrew Young, who was often dispatched to negotiate with white leaders because of his nonthreatening demeanor.
Signature lines: "Andy, when the Klan finally gets you, here's what I'll preach: 'Lord, white folks made a big mistake today. They have sent home to glory your faithful servant, Andrew Young. Lord, have mercy on the white folks who did this terrible deed. They killed the wrong Negro. In Andrew Young, white folk had a friend so faithful, so enduring, they should never have harmed a hair on his head. Of all my associates, no one loved white folks as much as Andy.' "
What others say: "King lived under the tremendous burden of people's expectations," says Jonathan Rieder, a sociology professor at Barnard College in New York who includes King's mock eulogy and other examples of his "backstage" personality in his book, "The Word of the Lord is Upon Me: The Righteous Performance of Martin Luther King, Jr."
"By temperament, he had a tremendous sense of refinement and dignity," Rieder says. "The burden of dignity was always being proper because he was leading people. So this was a special moment with his preacher buddies when he could cut loose and renew his spirit to go on."
Letter to Coretta
Written on July 18, 1952, to his future wife, Coretta Scott, in which King revealed some surprising thoughts on capitalism and communism.
Why it's important: There's a theory that King adopted more radical economic theories in the last three years of his life. But King's 1952 letter reveals he was radical far earlier than most people realize.
What he said: The letter is an intriguing mix of the personal and abstract. King woos his future wife by telling her: "My life without you is like a year without a springtime which comes to give illumination and heat to the atmosphere which has been saturated by the dark cold breeze of winter."
He then switches gears and starts praising a recent book on economics he has read. He says he would "certainly welcome the day to come when there will be a nationalization of industry ... and a better distribution of wealth."
Signature lines: "I imagine you already know that I am much more socialistic in my economic theory than capitalistic. And yet I am not so opposed to capitalism that I have failed to see its relative merits. It started out with a noble and high motive, to block the trade monopolies of nobles, but like most human systems it falls victim to the very thing it was revolting against. So today capitalism has outlived its usefulness. It has brought about a system that takes necessities from the masses to give luxuries to the classes."
What others say: "King steered toward socialism early on," says Michael G. Long, author of "Christian Peace and Nonviolence: A Documentary History."
"He's speaking about the need for the demise of capitalism, the need to nationalize industries. Early on he has this dream that equality won't happen in America until there's a radical redistribution of wealth."
'Where Do We Go From Here: Chaos or Community'
King's fifth book was published in 1967
Why it's important: This is King's last -- and most radical -- book. By 1967, he was organizing a "Poor People's Campaign," a plan to dispatch an interracial army of poor people to occupy Washington and force the U.S. government to address poverty.
What he said: He takes on black nationalists who ridiculed nonviolence. He says the passage of civil rights laws is not enough. The country must institute a "massive, new national program" to attack poverty. He predicts the civil rights movement will go international as oppressed peoples in other countries adopt nonviolent tactics to combat America's "economic colonialism."
Signature lines: "White Americans must recognize that justice for black people cannot be achieved without radical changes in the structure of our society. The comfortable, entrenched, the privileged cannot continue to tremble at the prospect of change of the status quo. ... This is a multiracial nation where all groups are dependent on each other. ... There is no separate white path to power and fulfillment, short of social disaster, that does not share power with black aspirations for freedom and human dignity."
What others say: "I get so tired of people turning Dr. King into a dreamer," says Doreen Loury, a sociology professor at Arcadia University in Pennsylvania, who says she was blown away by the book when she first read it in the 1960s. "They made him safe. He was a revolutionary."