I've got 'Homicide' laying on my desk as we speak and I am plannig on reading it as soon as I've finished the show.
It had humour indeed, but IMO It's very dark and cynical humour, as the characters (especially the cops) ask themselves what the point is of what they do, because it is all so corrupt and 'the game never changes'.
Definitely, that is a big part of the show.
As for Homicide, the book is good from what I can remember. The show is a little tedious though. Being an episodic procedural, it's not much like The Wire.