The great majority of this book is a detailed, step-by-step account of the how the author constructed a C-language big-number library. It treats wrapping the C-libraries with C++ classes as a separate chapter -- nice touch. This treatise is thorough, well-written, and engaging. Nicely done. It is written from a mathematicians perspective, and its mathematical underpinnings may repel some programmers, but they need not fear. In the end the C-code is well-explained.
The only "cryptography" of note is a chapter on implementing RSA with the library. Indeed, the great majority of implementing RSA and similar ciphers is creating the big-number libraries. The RSA chapter is, fortunately, much more than a simple example. It is an excellent treatise of the pitfalls in implementing RSA. That is, RSA is more than a "simple example." If you are implemeting RSA with any big-number library, this chapter alone is worth the cost of admission.
There is a chapter on AES (Rijndael) cipher, but it is out of place here and an obvious paste-on, probably a last minute, hot-button item required by the publisher. It really has nothing you can't get from the original NIST documents on AES.
I'm willing to believe that the title and AES paste-on are the fault of the publisher, because otherwise Welschenbach did an excellent job -- lots of hard and careful work, well documeted. With a title-change, I'd give it a 5-star.
The question in the end is: Why would you want this book? Apart from general interest, the audiences that come to my mind are those who are constructing such libraries on their own, and those who need to use the software included with the book (on a non-commercial basis -- the software license prohibits commercial use).
This book covers everything you need to know to write professional-level cryptographic code. This expanded, improved second edition includes about 100 pages of new material as well as numerous improvements to the original text.
The chapter about random number generation has been completely rewritten, and the latest cryptographic techniques are covered in detail. Furthermore, this book covers the recent improvements in primality testing.