It is a fantastic book. It isn't (only) a reference book though. For me it was the best way of learning the maths used in my physics degree.

The book has many worked examples, and the extensive end-of-section questions have the answers in the back of the book (for every 2nd question). This means you can learn by "reading then doing", and see if you have got the answers right - something many textbooks lack.

When I try to learn from other technical books, I often find myself thinking "I wish they'd written this in the same style as Boas".