This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
There are some things, like the "length" pseudo-instance-variable in arrays, or the "class" pseudo-static-variable, or the "this" member, or the "super()" method, that are more part of the language than part of the API, and this is one of them. The primary source for stuff like this is the Java Language Specification, browseable free online in HTML form. The third edition includes enums and other features introduced in Java 5.
Now, reading the JLS is not for everyone -- many people get this information "secondhand" through various Java language textbooks. But the JLS is the go-to book for deciding arguments and impressing your friends.