A nice article about standard ANSI joining syntax:

http://www.oracle.com/technology/oramag/oracle/01-nov/o61sql.html

Recognizing well known patterns involving outer joins will form a ground-level vocabulary for your further explorations of your data. This article examined syntax, surprises, and rules of thumb with respect to the usage of joins.

http://www.onjava.com/lpt/a/4443

A blog with nice graphic show how join work - http://www.codinghorror.com/blog/archives/000976.html