"Best" is a vague metric, but one textbook I am always happy to recommend is the Princeton Companion to Mathematics. The book is a selective review of the most

"Best" is a vague metric, but one textbook I am always happy to recommend is the Princeton Companion to Mathematics. The book is a selective review of the most significant ideas in maths and mathematical research. The great editorial process is what sets this book apart from the rest: each chapter is authored by an outstanding expert in his/her particular field (among which several fields medalist the likes of Terence Tao) and the editing by Timothy Gowers does an amazing job of uniforming the tone, notation and rigor across the whole text. The authors were evidently chosen also for their communication skills since the articles come across as quite discoursive and really convey the beauty of modern maths. In the book you'll find not only the articles on mathematical concepts, but also an introduction to modern mathematics, chapters on the relationship of maths with other sciences, a thorough history of mathematics and a collection of biographies of famous mathematicians. Well recommended to anybody with an interest in maths. My background is in CS and I found it approachable (except minor sections) and illuminating!