Apologies for the self-plug, but you might find some stuff in the later-stage Aggregates and Recursive exercises on http://pgexercises.com . There's a bunch of stuff on CTEs and window functions in there.

I like Date's SQL and Relational Theory (http://shop.oreilly.com/product/0636920022879.do). It won't tell you about the latest changes to the standard, but it will help you get a handle on the relational model, which is the key to using SQL intelligently.