It's good to see high-quality books on open source software, and this one is well-organized, thorough and profusely illustrated. It happens to make a great online GIMP tutorial as well.
Note: as you might expect, many of the Web pages that make up the book are image-heavy (as you might expect), so if you're on a slow connection, browse the detailed, outline-format table of contents carefully.
And if you do have the bandwidth, you can slurp down the entire book to browse later. When's the last time you read a book that came as an HTML tarball?