The main thing that you gain by reading the book (and working through the puzzles!) is a deep knowledge of the traps, pitfalls, and corner cases in the Java platform. The behavior of the little programs that comprise the puzzles will surprise you, even shock you. Almost every puzzle corresponds to a subtl e bug that could bite you. Once you've solved the puzzle, you know the trap, and it becomes much less likely that you'll fall into it. In that sense, solving the puzzles should make you a better programmer.
Also, we tried very hard to make the book entertaining. It's filled with optical illusions, jokes, pop-culture references, and the like. So hopefully you'll gain tons o' fun by reading the book.
Finally, if you're a language designer or a student of language design, you'll gain insight into how certain language (and library) design decisions can result in traps and pitfalls in the platform.
Click and Hack, the Type-it Brothers
Joshua Bloch <br />Author of <a href="http://www.amazon.com/exec/obidos/ASIN/0201310058/ref=ase_electricporkchop" target="_blank" rel="nofollow">Effective Java</a> and coauthor of <a href="http://www.amazon.com/exec/obidos/ASIN/032133678X/ref=ase_electricporkchop" target="_blank" rel="nofollow">Java Puzzlers</a>