Great Book

I'm reading City of God, which was written by Saint Augustine in 1467. It amazes me how relevant his thoughts are even all these years later (I'm no good at math, but isn't it 542 years?)He explains how the pagan and philosophical beliefs of his time, and earlier, were false, and how Christianity is different. The book is over 1,000 pages long, so I'm probably not going to be done reading it for years, but it's the kind of book I enjoy flipping through for gems of knowledge and points to ponder. His musings on Good and Evil made a big impact on me today, as he says basically what I've been trying to express in the novel I'm working on--only he sounds smarter. I won't go into detail about it since it would take a lot of background explanation, but I will say it has me fired up to finish writing my book. I don't expect mine to have the lasting impact of Saint Augustine's, but that's OK. It will be shorter and easier to read.

Does anyone else have a book to recommend that makes a real impact on their thinking?