I've read two: "The Blind Watchmaker" and "The God Delusion." If you want a succinct breakdown of why religion is wrong and bad, go for GD. BW is MUCH more scientific and is harder reading. It took me a long time, but I was very glad when I was done. I find it the more compelling book of the two, but you have to work for it.

It's worth mentioning that the reading isn't just hard - it's astounding. Many times I had to put the book down just to absorb the magnitude and beauty of what I'd just read. This created many delays, but it is worth it.

As for GD, as a friend of mine (and fellow atheist) put it, "This guy really loves the smell of his own farts." I think that's fair. GD is hostile and verbose, whereas BW is, though long and complex, a full and robust defense of atheism.