Roy Osherove’s : The Art of Unit Testing

Read it and LEARN !

Andrew Hunt & David Thomas’ : The Pragmatic Programmer

Lots of stuff to improve behaviour, code quality and probably also self esteem when you recognize some of your own behavioral/coding patterns.

Martin Fowler et al’s : Refactoring

The basis of all modern software development and maintenance.

Erich Gamma, Richard Helm, Ralph Johnson & John Vlissides : Design Patterns

An important part of the foundation for every software developer.