The second edition of Effective Java (by Joshua Bloch) is now updated for java version 6. This book is a must-read for any serious java-developer. Contains very practical and up-to-date tips for writing great java programs.
Checkout the effective java site.
The first edition -- still valuable but a bit outdated -- is available on google
Joshua Bloch presents the new version of his book on parleys.com. Highly recommended: