Here are some agile books that I recommend:
- Extreme Programming Explained by Kent Beck is the original XP manifesto.
- The Art of Agile Development by James Shore is a comprehensive reference for agile/XP.
- Clean Code by Uncle Bob is a detailed manual for producing high quality code, right down to variable naming.
- The Art of Unit Testing (with examples in .NET) by Roy Osherove is the only good book I've found specifically on unit testing.
- Working Effectively with Legacy Code by Michael Feathers is an interesting guide to bringing legacy code under test and under control.
- I'm currently reading Agile Estimating and Planning by Mike Cohn, which covers story points, sprint length, agile planning and a lot of other topics that are important for management to understand as well as developers.
- Two very reputable books on use cases and user stories are Writing Effective Use Cases by Alistair Cockburn and User Stories Applied by Mike Cohn, but I've read neither (yet).