Bookhounds aside, I've been very interested in Trail of Cthulhu and the other GUMSHOE systems for some time now. I've never run an investigative game before, but my wife has been asking for something with less goblins and more murder mysteries. While I'd probably stick to Unknown Armies for supernatural-themed games, it might be possible to strip Trail of Cthulhu down to a mundane detective game.
Also, these are Kenneth Hite books. No one RPG collection can be harmed by the inclusion of more Kenneth Hite books.