http://www.goodreads.com/book/show/18956.Homicide
I wish it were required reading in at least all journalism school curriculums. It's a book about death and the most "exciting" part of police work but also one that reveals the banality and politically-motivated mechanics of such a system. I almost hesitate to call it "thought-provoking" because it, among other things, is just a great page-turner, though astonishingly completely non-fiction. I say "astonishingly" because Simon, according to him and the detectives he wrote about, had pretty much free reign to write what he wanted...there's no way such a book could be written today, as the police tactics that Simon describe, out of context, would likely cause a lot of outrage today.
As a bonus, reading "Homicide" will make all of the jargon in "The Wire" much more understandable. Many of the memorable incidents in "The Wire", including the very first scene, and scenes like the photocopier-used-as-lie-detector, come straight from "Homicide"