This book is brilliant. An overview of all of mathematics, insofar as that's possible. High quality articles. For instance Terence Tao wrote the entry on Compactness and Compactification (topology/analysis) among others. I've had the print volume for a few years now, highly recommended.
It covers only pure mathematics. From the Preface:
"... it was suggested that a more accurate title would be "The Princeton Companion to Pure Mathematics": the only reason for rejecting this title was that it does not sound as good as the actual title."
Worth noting for the interested reader that's disappointed upon reading your comment that there's also a Princeton Companion to Applied Mathematics (ISBN-13: 978-0691150390, ISBN-10: 0691150397).
As an aside, from time to time I try to cut the filename out of these kinds of urls and I'm delightedly nostalgic whenever I can browse the server, as it was so common some years ago; these were the kind of "secret powers" that attracted a lot of kids to learning about computers...
