These are books* and resources I have found to be useful:
- PragPub Magazine, a monthly publication from The Pragmatic Bookshelf.
- Spring in Action, Third Edition
- Effective Java (Second Edition)
- Groovy In Action
- Practices of an Agile Developer
- Programming Ruby (2nd edition): The Pragmatic Programmers' Guide
- RESTful Web Services
- The Pragmatic Programmer: From Journeyman to Master
- Programming In Scala 2nd Edition
- Working Effectively with Legacy Code (Robert C. Martin)
* N.B. Some of these links are affiliate links.