In my case it so happened that I started discussing faith with a long-time online friend whose spiritual views I didn't yet know, and he turned out to be a Christian with a high regard for the Bible, who also has an interest in science. As our discussion turned to our readings on spirituality, I acknowledged (I think it was me) that I probably spent more time on books that reinforce my point of view than on books that challenge it, perhaps a case of confirmation bias. (I've been exposed to many poor arguments for Christianity, and dismissed them; but possibly that was largely a function of having started out with that bottom line already written and picking arguments I wouldn't have much trouble refuting.)
In the spirit of experiment we agreed to a "trade" - he would read (thoughtfully and with an open mind) a book of my choosing on reasons to doubt faith, and I'd do the same with a book he chose on Christianity.
So the idea here is to pick a book that's the "best argument from the other side" (as in quote 3 here).
I recommended The God Delusion - I'm not sure if that's the best choice given the above intent, but it's what came to mind on the spot.
Would you make a different choice? If so, what?