Back when XML was starting to become popular, I figured I'd better look into it and picked up a copy of "Mastering XML" by Ann Navarro et al. Despite that book having dozens of errors in, it managed to last me quite a few years - there is a lot of information in there that can help you further your learning past basic XML concepts.
If you had trouble with this chapter and want to try something easier, I can heartily recommend the book XML Step by Step by Michael Young - it is chatty yet informative, and should have you up and running with XML after just a few hours of reading.
There's a gentle (but quick) introduction to XPath at www.w3schools.com/xpath/default.asp .
Copyright ©2015 Paul Hudson. Follow me: @twostraws.