Salesforce developers actually have a great resource on Developer Force - http://developer.force.com/ - but I wanted to add some of the books Salesforce has released to this list to share with everyone. Salesforce's languages are Apex (similar to Java) and Visualforce (similar to HTML)
* [Apex Design Patterns and Best Practices](http://www.gobookee.org/get_book.php?u=aHR0cDovL3d3dy5zdW5kb2dpbnRlcmFjdGl2ZS5jb20vd2hpdGVwYXBlcnMvU1VOX0NodWNrX0FwZXhkZXNpZ25wYXR0ZXJucy5wZGYKQXBleCBEZXNpZ24gUGF0dGVybnMgYW5kIEJlc3QgUHJhY3RpY2VzIC0gU3VuZG9n)
* [Introduction to Programming Using Java](http://math.hws.edu/javanotes/) - David J. Eck
* [Java Application Development on Linux by Carl Albing and Michael Schwarz (PDF)](http://www.phptr.com/content/images/013143697X/downloads/013143697X_book.pdf) (PDF)