I'm not entirely sure what book I want, even what subject it would be considered as...which is why I'm here. My mathematical knowledge is dismal. I want some kind of book that will cover the very fundamentals rigorously. I don't want a high school text book which will just bs its way around, but something that will be challenging and force me to think in mathematical ways. I'm seriously deficient in geometry and number theory, so I'm kind of leaning in that area. I've looked at the table of contents of the book "What is mathematics" by courant, and I like what I see. Is this a good book to get?