This was on the "staff picks" shelf at Powell's a while back, and I wrote it down as something that sounded interesting. The idea is this: Gabriel Black is in prison for life, for a murder he didn't commit. He takes up reading, and comes across a book by a man who's cured of cancer and gives evidence of the existence of God. Gabriel decides that's too easy, finding evidence of God when your life turns out wonderfully, and starts his own God file. He reasons that the real test is whether he can find evidence of God in a hellish place like prison.
The book is basically a series of not-necessarily-connected vignettes, thoughts, and other bits of writing that Gabriel has put into his file. Some are tales of people in prison with him, some are letters to his ex-girlfriend, and some are just memories or meditations on things from suicide to the sound of a bird chirping. The concept sounded really great, but in reality I found it to be a little weak. In part, I suppose it's because I kept thinking of "Shawshank Redemption" (I've only seen the movie, haven't read the book), and this book is no "Shawshank Redemption."
It's a short book, and wouldn't take too long to read, but I wouldn't put it at the top of your list.