Good Omens is a must-read if you like Supernatural (circa Seasons 4-5). I think a cursory read-through of the book's wiki page would clear up any confusion you may have about most of the crossover fics.

All you have to know about Leverage to get my story is that Eliot Spencer (played by Christian Kane) is a retrieval specialist, which means he gets hired to steal/retrieve things. And he's really badass.