xPath

In first place i thought it will be simple, in a way boring. Well not really.
Offcourse www.w3schools.com/XPath/default.asp is begining. Lot more found on http://www.xfront.com/xpath/ . I have used 2.0, though 1.0 and 2.0 is available on the website.
I have used http://try.zorba-xquery.com/ to run and get output of xPath expressions. You can have tools like Stylus, Altova, Oxygen etc or may be some online tool.

Couple of things i have noticed to remmember and get good hold of:
Sequence
Item( can be Node of atomic value)
Functions(over 100 built-in functions) includes String, dateTime, numeric and other functions
Operators
General comparision operators(=, !=, <, <=, >, >=)
Value Comparison Operators(eq, ne, lt, le, gt, ge)
Arithmetic Operators(+, -, *, div, mod, idiv)
Ueages of Regular Expressions in xPath
Predicates
for expressions
If-then-else expressions(you cannot miss else in this kind of expressions, for no action you can use else ())

xQuery functions and Operators are strongly typed. "instance of", "castable as", "cast as" operators are available(equivalent operators are available with JAVA).

Enjoy Learning!!!