I've read a few books now on usability, accessibility, navigation design, etc. Some of these, such as Steve Krug's "Don't Make Me Think" were excellent, and really changed the way I looked at web design. Nevertheless, this book still managed to floor me -- throughout, I was exposed to critical design concepts that I had never even heard of, and to new and deeper perspectives on those that I already knew of.
One thing that I especially liked about this book, is that in contrast to many other books or articles in this genre, Kalbach provides scientific research to back up his claims about what is and isn't usable. Many other books will make claims that something is a best practice, and back it up by citing other people who say so, but won't back their claims up with actual data. Many of the studies that Kalbach contradict "popular wisdom" on usability in areas such as the usefulness of breadcrumbs, or the appropriate length of anchor text.
I also enjoyed the questions/exercises at the end of each chapter. The book is extremely information-rich, with many deep concepts packed into each chapter. It was nice to give my brain a refresher at the end of each chapter, to make sure that I had retained everything.
I literally cannot think of a single problem that I had with this book -- it was accessible, in-depth, well ordered, and aesthetically pleasing -- I could hardly put it down. There are many other good books out there on these topics, but none of them that I've found has managed to pack as much useful information between the covers as this one, and some of the topics, I've seen covered nowhere else.
This is how computer science texts should be written! A+++.