Definitely worth reading, but you should be aware precisely what it is you are buying. This is an in-depth look at the SSL and TLS protocols -- their history, ideology, and function. Related topics are discussed only briefly. For example, if you are buying this book because you want to be an expert on SSL certificates, you may be slightly disappointed to learn that the subject is treated in only one chapter, at the end of the book. You will get a good foundational understanding of why certificates are important, and some details about how they work, but little will be said about practical subjects, such as how you create them, where you get them signed, what products are the best, and so forth. Likewise, the book purposefully avoids discussing details about specific implementations of SSL; so you won't learn much, for example, about the capabilities of various Web browsers, or how to set up OpenSSL on your Linux server.
Having said that, this book provides a very detailed description of the protocols, so you will gain a good working knowledge of all the terminology, as well as what happens during the actual encryption and authentication. This will give you a good conceptual context for reading elsewhere about subjects such as encryption cyphers, authentication procedures, HTTPS security, and SSL exploits.
The buyer should be aware that the author has a rather dry and condensed writing style, and that a lot of material in the book is couched in mathematical language. In chapter 2 especially, the basics of cryptography are explained almost entirely in set notation and are illustrated with austere block diagrams that really do not illustrate much. The author is also fond of overusing a number of quaint phrases: for example, he will frequently write "It goes without saying..." completed by some statement that you would never have known if he had not said it.