This is basically a "everything you need to know about APIs and mashups" kind of book. It is very example-focused, so besides giving the foundation for understanding alphabet soup like REST, SOAP, XML and JSON, you also get to follow some complete projects that use these acronyms for some healthy learning-by-doing. Here's a basic rundown of the things you will learn: XML, RSS, RDF, XSPF, XML-RPC, REST, SOAP, JSON, SPARQL, some basic Ajax, screen scraping with DOM functions, installing and using PEAR classes, and creating a simple proxy for cross-site XMLHttpRequests. The mashup examples use all of these, and they are very practical projects.
I learned about XML/RDF/REST in college with Java as the language of choice, and I've been able to use some APIs in the past with the help of PHP client classes. This book, however, covers a lot of different technologies in less than 300 pages, and it gives you what you need to know to not be dependent on client classes provided by the myriad of services out there, which is extremely helpful since you can't always expect a drop-in client class to be available.
And even if you think that as a web developer you have no plans of ever building a web application or mashup service, there's still the chance that your employer or some client will someday need one of these services on a project. In that sense, the things taught in this book are pretty much required knowledge for any web developer these days, because as much as web 2.0 might be a fad, all the alphabet soup technologies involved are not. We'll be using them for many, many years to come.
In short, I recommend this book. Even if you know this stuff pretty well, this book still offers a lot to learn.