- E.L. James' "Fifty Shades" books have sold 31 million copies worldwide since March
- Insiders say books benefited from perfect storm of smart marketing and e-book mania
- Some credit trilogy's tame covers with bringing in new audience of erotica readers
- Hoping to mimic its success, publishers reissue erotica titles minus sensual covers
(CNN)It's a formula that's been tried and tested in countless erotic novels in recent memory.