I frequently am asked what is a good book on XML. This is a difficult question to answer because XML is such a huge topic and it really depends on the specific area you are interested in. The other challenge is that many books include chapters on XML topics that are no longer relvent for 99% of the world. Books that include sections on DTDs should be avoided. I also try to steer people clear of any book that recommends XSLT. I have found that XQuery typeswitch functions have proved to be better for most document-oriented transforms. So with this warning, here are my core XML books.XQuery by Priscilla Walmsley
This is a must-have book. It is a great reference manual for the XQuery language and is very well written.
XPath 2.0 Programmer's Reference by Michael Kay
This book covers XPath. XPath is used in XQuery but most people will never need to know the complexities of XPath 2.0. Get this book only if you are very serious about writing very complex XQuery expressions.