Hashing is vital in many computational applications. It forms the backbone of searching, for example. So this book is very useful in giving a detailed explanation of the theory behind various hashing methods.
It is slightly dated. It does not describe the Simple Hashing Algorithmn 2, which is currently considered the most secure method. But the ideas in the book are still applicable. Useful if you want a rigorous understanding of the theoretical underpinnings of the field. Or if you want to start designing your own hash algorithm.