Ordinarily, I wait until I've read all or a lot of a book before I rate it.  I am not very far in but am more than happy to give the book 5 stars for the great benefits and insights I have already received from it.  Initially, I thought this might be a book with some nice illustrations while not all that substantive on the mathematics, and I was willing to buy it even then.  (I'm reminded of that great canard that every book on topology either ends with the definition of a Möbius strip or is a personal communication to J.H.C. Whitehead.)  Here, what a pleasant surprise to see the breadth and depth of the topics!  And yet, written so naturally and agreeably that this book should be accessible to undergraduates.  An outstanding example of what mathematics and mathematical exposition should be.