This is an excellent book. It's very well written and covers just about everything you need to know about this emerging and very important topic from what tagging is to setting up a tagging system of your own. I particularly liked the fact that the author sees tagging as complementary to the established disciplines of taxonomies, controlled vocabularies and facet analysis and righly affirms that each has their place in the information space. Tagging, as another form of metadata (which the author describes as 'documentation for your data'), takes its place as a very important addition to these methods. The key is knowing when to use which or when and how to use them together.
The first few chapters introduce the subject by describing what tagging is, its value and system architecture citing real world examples - always a good move. There then follows a very good discussion on metadata and classification schemes. This is most welcome as many books tend to contain too much technical detail on these subjects that only appeals to specialists. The author keeps it simple but accurately describes the subject with clarity and precision. If you're not really sure what taxonomies, controlled vocabularies, facets, folksonomies or metadata in general are then you will be after reading chapter 4. To me this (and the following chapter dealing with visualisation and navigation or 'tag clouds') are the pivotal chapters in the book as they set tagging in context with the other classification tools and explains why they're so important. The remaining chapters cover user interfaces and designing your own tagging system and nicely rounds the book off. There are also 3 very informative case studies which consolidate the topics covered in previous chapters.
If you know nothing or very little about tags (or content organisation tools in general), this is an excellent book and, as the material is so lucid, may well be the only one you'll ever need. If you already know a fair bit about the subject but want a book that brings it all together, this is also an excellent purchase. The author and publisher have done a great job balancing the book's length and depth. The result is an enjoyable, thought provoking and informative read. You'll want to refer to it again after you've read it.
One of the nice touches of the book is that each chapter begins with a "What you'll learn in this chapter" and ends with a summary. This is very helpful and adds to the impression that you are actually learning something. As welcome as this is, the book would benefit further from the inclusion of a glossary, an index of illustrations (there are many good illustrations) and a back of book bibliography or further reading list. Further reading references are interspersed thoughout the chapters but it would be nice to have them all together at the end of the book. Perhaps these will be included in a future edition?
All in all though, the book really can't be faulted. Highly recommended.