Quite possibly the best math textbook I've ever seen:
The authors take on the material in short, logical, easily digestable installments that guide the reader through an incremental development of ideas that is easy to follow and truly a pleasure to read.
The discussions consist of thorough, step-by-step explanations, without leaps that make presumptive or undue expectatations of readers' knowledge and capability. These are complemented by clearly presented and well illustrated examples.
There are frequent and extensive exercises, far beyond what is usual in math texts (providing the usual answers to odd-numbered questions, of course).
A brilliant textbook by any measure.