I got a very nice welcome from Nils Berglund to the world of BLOGs. Thanks Nils!
However, at least for now, the XQuery book that I co-authored (XQuery from the Experts), is not only one of the best, it is THE best book on XQuery :-). Check out its sale rank history. You will see that you should go out and buy it to get its rank back up :-).
Seriously, this book gives a very good overview over the history and design backgrounds of XQuery. It features a tutorial, theoretical foundations, and how XQuery relates to several technologies, ranging from XSLT over relational databases to native XML databases. All chapters have been written by members of the XQuery Working Group that have provided large contributions to the language. It is NOT an XQuery reference book (check out Michael Brundage's book that will be published soon), but gives lots of good background material and how XQuery will relate to relational database systems. Before I stop touting my own horn, check out the Amazon reviews.
Since I wanted to promote the book a bit :-), and I also just recently signed up for the Amazon Associates program, I wanted to add a link to the blog's link list using the HTML code generated by Amazon (see end of the blog). Unfortunately, this totally screwed up the link manager of my blogging webinterface. Now I pray that Donny will fix it... :-)
Here are the Amazon links: