I've been going through the newly published Mozilla development book Rapid Application Development with Mozilla by Nigel McFarlane. I thought I'd add my comments.
It's very detailed. It provides detailed descriptions of XUL, XBL, RDF, templates, and so forth. For someone looking for detailed information about how to use these technologies and a bit about how they work, the book provides lots of information.
To some degree, the extra detail is also problematic. In many places, the book goes into more detail than is really necessary. There's a lot of text there. And, I think, a disproportionately small number of examples. If you were to go through the book attempting to learn XUL, I'd imagine you would be overloaded with information. Although I'm probably a little biased, I think the XUL tutorial is a better way to learn XUL, while the book is quite good for a greater understanding of the details.
I had hoped instead to see a much greater amount of focus on XPCOM and the available components. The book does provide, in the XPCOM chapter, some detail about the RDF APIs as well information about file and network reading, and provides an introduction to a number of other things, such as sockets, security and preferences. But, considering the detail in earlier chapters, this is too little I think.
However, overall, I think Rapid Application Development with Mozilla is a good addition to the Mozilla/XUL developer's library.