The second half explains PGP usage and where you can find it online. Unfortunately, a lot of this seems dated -- however, to be fair, the book is over five years old. You'll probably be better off with another resource such as the included documentation.
O'Reilly's PGP book can be divided into two sections. The first section is really a history of cryptography and how PGP fits in this context. I found this section surprisingly enjoyable as you learn about the long and tortuous struggle between the NSA and people who want to promote freedom and privacy. On a more concrete level though, you do learn quite a bit about different encryption algorithms and key algorithms, such as the RSA and Diffie-Hellman as well as other concepts important to cryptography. Admittedly, the history itself makes for pretty interesting reading.
The second section is about PGP usage, and it is very thorough in its coverage. You will learn just about every possible feature in PGP, and how to apply them to a number of possible situations. I like reading this book over the PGP manuals just for the time and care put into it, if not the amusing examples.
One thing other reviewers have rightly touched on is the age of the book. TIme has passed. The RSA algorithm is now free and open, and PGP clone called GPG is now in wide use. I am definitely excited to see a 2nd edition of this book in hopes that it will cover such things.
However, regardless of the age, this book is an excellent primer into PGP and cryptography culture, and newbies like me will certain enjoy reading it.
Garfinkel's book is extremely basic. It covers the same ground as the PGP documentation, but not as well. Worse, it's badly out of date by now.
A much better bet is to read the online documentation for GnuPG, the free PGP clone, at www.gnupg.org. If you use UNIX, you should use GPG instead of PGP anyway: PGP has a wonderful interface under Windows, but has really stagnated for UNIX users.