Mere Christianity was not quite what I was expecting. I found myself arguing with Lewis all the way through the book, but when I got to the end I had to admit that I agreed with his basic statements. I suppose what made me argumentative was the knowledge that if I didn't already believe what he was writing about, I seriously doubt I would have been convinced by his arguments. Particularly since a large portion of his arguments consist of "What if the world was really more like [insert analogy] instead?" which is interesting, but not exactly irrefutable logic and made me want to ask, "Or what if it's really more like a kumquat?"
I can't, however, think of many other books that give such a concise and quotable summary of basic Christian beliefs. Below are a couple favorites:
I feel a strong desire to tell you--and I expect you feel a strong desire to tell me--which of these two errors is the worse. That is the devil getting at us. He always sends errors into the world in pairs of opposites. And he always encourages us to spend a lot of time thinking which is the worse (160).
Do not imagine that if you meet a really humble man he will be what most people call "humble" nowadays: he will not be a sort of greasy, smarmy person, who is always telling you that, of course, he is nobody. Probably all you will think about him is that he seemed a cheerful, intelligent chap who took a real interest in what you said to him. If you do dislike him it will be because you feel a little envious of anyone who seems to enjoy life so easily. He will not be thinking about humility: he will not be thinking about himself at all (114).