What is an excellent XPATH reference for learning correct usage of the syntax?

XPath is something important and one should not be misled by incomplete and (at least partially) incorrect sources.

This is why I recommend the books by Michael Kay -- his XSLT 1.0 books have good coverage of XPath 1.0. His XPath 2.0 book is the best XPath 2.0 reference.

For XPath 1.0 another excellent resource is Jeny Tennison's book "XSLT and XPath on the edge".

For links and book covers see my answer to another question.

Important: Nothing can substitute learning by practice and here is where The XPath Visualizer is your friend.

Zvon.org worked pretty well for me. I pretty much learnt XPath (and XSL) from scratch using that site. They've got a bunch of tutorials and very readable examples.

I suggest the Mozilla Developer Network: https://developer.mozilla.org/en/XPath

MDN is my first stop for most things web.

Note: Stay away from W3Schools! (See: http://w3fools.com/)

There is XPath 1.0, and then 2.0, which is really much more sophisticated and powerful. XPath is mostly used as an embedded language in XSLT and XQuery. The XPath chapter of Evan Lenz's XSLT 1.0 Pocket Reference is good to get you started on 1.0. Any XPath questions you might have will be answered on the XSL-List, where many experts hang out, and which is where I've learnt the X-stuff.