My bookshelf at work just got about five pounds lighter with the addition of this book... HTML & XHTML Pocket Reference (3rd Edition) by Jennifer Niederst Robbins.
Contents: HTML and XHTML Fundamentals; Alphabetical List of Elements; Character Entities; Specifying Color
This is a great pocket guide, and exactly what I look for in this type of book. No fluff, just well-documented information that's easy to find, with a small number of examples to show you the format. I really appreciated the documentation on which elements and parameters are deprecated. This comes in really handy if you're looking to code strict XHTML, but you're unsure as to whether a certain feature is going to be supported or not. In most cases, I know the general tag I want to use, but I might be a bit confused as to the exact format of the different arguments. With the pocket guide, I can find that tag in seconds, see the options, and move on. I love it.
The book I've been keeping on my shelf at work for HTML reference is one of those five pound doorstops that covers absolutely everything. The problem is that I have to check the index to find what I need, and I end up using a different book for CSS information. With this pocket guide, I can retire that book, gain more room for other titles, and give my poor shelf a bit of a rest... :)