The 25th Hour - David Benioff
Like Robyn, I also wanted to finish the book before we saw the movie, which we did last night. Although the story focuses primarily on Monty, the drug-dealer who's going to prison in 24 hours, you do get to know his two best friends as well: Jakob Elinsky, now a school teacher, and Frank Slattery, a Wall Street trader-type. Having seen the movie, I'm glad that I'd read the book first—some of the scenes were probably easier to understand since the movie is less suited to explaining a person's motivations. Also, in the book (written pre-9/11), Monty is constantly thinking about how he would've made a great firefighter, but they took that out of the movie—it would have seemed trite and contrived in the post-9/11 New York City, with its changed skyline and a memorial to the firefighters at Monty's dad's bar. I disagree with Robyn's statement that the book has no plot and therefore won't spoil anything for the movie—there are some bits here and there that you don't know at the outset: who turned Monty in, for instance, or what sort of plan he's trying to cook up before his 25th hour comes.
It is definitely a guy story, and also a New York story—Benioff refers to this landmark or that street in a way that wouldn't bother me at all if it were an imaginary city; but since I know that it's actually in New York, I wish that I'd actually been to these various places, to know what he's talking about. No, it doesn't have a typical plotline; it uses flashbacks and little incidents to tell a story about friendship and bad choices and dealing with those consequences.
My recommendation: read the book, then see the movie. They complement each other nicely.