One of our great teachers here at MarkLogic, Ruth Stryker, recently reminded me of some quick reference (aka cheat-sheet) cards that were put together by Dan Vint. He's got a few resources at his site that I like, but the two that seemed most useful are:

• XPath 2.0
• XQuery 1.0 and XPath 2.0 Functions and Operators

I've left links to these in the sidebars of our docs and learn sections should you misplace them.