I always have trouble with what-is-your-favorite questions, because there are usually so many candidates to choose from. That is especially true in the case of religion. For my early years, it was Bertrand Russell's
Why I am not a Christian, because that was the most popular book of its type when I was young. Nowadays, there are plenty of very good books, but I'll just mention two of my favorites:
Stewart Guthrie's Faces in the Clouds.
Anthropologist Stewart Guthrie explains why theism exists--because of our inborn tendency to anthropomorphize inanimate objects and forces. Dawkins discussed his thesis in
The God Delusion but was apparently unaware of the source.
Bart Ehrman's God's Problem.
Ehrman gives a detailed review of what the Bible has to say about human suffering, which was the issue that he claims drove him away from religious faith.
I liked both of these books not because they were the best written works I've read on the subject, but because they gave me interesting insights that I did not have before I read them.