The Art of Software Security Assessment: Identifying and Preventing Software Vulnerabilities

http://www.amazon.com/The-Software-Security-Assessment-Vulne...

In my opinion, the most helpful text for transitioning from development to security would be:

http://www.amazon.com/The-Software-Security-Assessment-Vulne...

Beyond that add in a bit of system administration knowledge e.g. in-depth knowledge of operating systems and networking, and you have everything you need to break many many systems!

I think a good start would be to read http://www.amazon.com/Art-Software-Security-Assessment-Vulne...