Simpson J.E. XPath and XPointer

O’Reilly Media, 2002. - 210 Pages. ISBN:978-0-596-00291-6Referring to specific information inside an XML document is a little like
finding a needle in a haystack. XPath and XPointer are two closely related
languages that play a key role in XML processing by allowing developers
to find these needles and manipulate embedded information. By the time
you've finished XPath and XPointer, you'll know how to construct a full
XPointer (one that uses an XPath location path to address document
content) and completely understand both the XPath and XPointer features it
uses.