I have been remiss about the Trail of Cthulhu Bundle of Holding.

The only reason I haven’t bought that particular collection of roleplaying game e-books is because I already own it all. Except maybe the music.  The important thing, though, is that all of the Trail of Cthulhu books listed there are excellent and they’re based around a rules-set that revolves around investigation and mystery solving the same way that other games revolve around combat. Worth your time to pick up on the cheap.