Updating the list to include free Salesforce/Force.com books

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)
Merge pull request #344 from jessealtman/patch-1

Updating the list to include free Salesforce/Force.com books