I'm looking to get one of my professors a book/gift as a thank you, as he's helped me a lot with my undergraduate studies over the past two years. I was thinking about getting him an interesting book, so I was wondering if anyone knew of some good ones (preferably physics/science related). He specializes in gamma ray astronomy and supernovas, and astrophysics in general. I thought about getting him a copy of Newton's Principia, but I'm not sure. Any suggestions?