To be honest, it is hard to recommend a single book. If you really want to understand philosophy, you must read many books. So you might want to start with Hume's Enquiry Concerning Human Understanding, Berkeley's Treatise Concerning the Principles of Human Knowledge, Descartes' Meditations and Discourse on the Method, as well as Kant's Critique of Pure Reason and Principles of the Metaphysics of Morals. These are by far the best epistemology/metaphysics books I have ever read. It might seem obvious recommending such well-known books, but they are well-known because they are truly phenomenal. Also, if you are interested in the development of mathematics, and in particular analysis, I would recommend The Analyst by Berkeley -- a remarkable book. If however, you do not have time to read them all, start with Hume's Enquiry Concerning Human Understanding and Berkeley's Treatise Concerning the Principles of Human Knowledge, and avoid Kant' Principles of the Metaphysics of Morals, and any of Kant's books because they are very difficult to read. You need to read each book at least twice to understand Kant.