A really nice 628 page textbook for Mathematics from Carnegie Mellon University. I like the Author's description "The goal of this book is to help the reader make the transition from being a consumer of mathematics to a producer of it. This is what is meant by ‘pure’ mathematics. While a consumer of mathematics might learn the chain rule and use it to compute a derivative, a producer of mathematics might derive the chain rule from the rigorous definition of a derivative, and then prove more abstract versions of the chain rule in more general contexts (such as multivariate analysis). Consumers of mathematics are expected to say how they used their tools to find their answers. Producers of mathematics, on the other hand, have to do much more: they must be able to keep track of definitions and hypotheses, piece together facts in new and interesting ways, and make their own definitions of mathematical concepts."