Lil Wayne -- When Drake Humps Your Chick ... Just Write a Book (TMZ TV)

Sure, Lil Wayne could have freaked out when he found out Drake banged his GF -- but instead he chilled ... mostly because he was locked up on Riker's Island at the time.

But now, Weezy's found a way to cash in on Drizzy's prolific penis -- by writing a book about it. There will probably be other stuff in it too, but ...