I attended some of Doug’s excellent sessions at the WebSphere conference in Las Vegas waaaay back in 2001 (actually a number of months before I joined IBM). I subsequently emailed him a couple of times, and ended up being invited to be a technical reviewer of the manuscript. It’s a great reference book, and has some excellent examples. I’d also say that Doug’s writing style makes it very easy to follow.
Since I subscribe to Doug’s developerWorks blog, I noticed that a second edition is due out, now updated for XSLT 2.0 and XPath 2.0. You can pre-order it already (Amazon UK | Amazon US). I’m sure this is going to be a worthy follow-up. If you work with XML technologies, go put it on your wish list.