Most mathematics books are very dry, and almost appear to be written for the people who already understand the concepts.
While not as complete or mathsy as a lot of the others I've read, this book goes to great lengths to demonstrate geometric interpretations, answering questions like, "What does a matrix 'look' like?", "What does a coordinate space transformation 'look' like?" and things of that ilk -- tremendously useful. This book helped me a great deal during university and gave me a much better understanding of the concepts.
I'm not really a fan of the source code listings in the book, as they're plentiful and not particularly to my taste, but the rest of the book is great.