This is a great overview of many key areas of modern mathematics.  The book is wonderful because it's not a typical "For Dummies" book.  It has some real mathematics in it, and many of the theorems and proofs are mathematical masterpieces that every Ph.D. student should know.  If you want to master the basics, I would use this book along with your text books.  This book gives a really good overview of many important areas of mathematics.