Extreme Programming Installed by Ron Jeffries, Ann Anderson, and Chet Hendrickson. An introduction to the nuts and bolts of Extreme Programming.
Ron Jeffries's blog contains many great articles about Scrum, XP, Agile, and life.
The Nature of Software Development Ron Jeffries's great book on why software is hard and what to do about it.
Working Effectively with Legacy Code by Michael Feathers. The go to book in how to rehabilitate big balls of mud.
Agile Retrospectives by Ester Derby and Diana Larsen. The best guide to creating a safe space for effective retrospectives.
Software For Your Head by Jim and Michele McCarthy. A very intesting set of tools for creating teams.
Fifty Quick Ideas to Improve Your User Stories by Gojko Adzic (Author), David Evans (Author), Nikola Korac (Illustrator). Some great ideas on creating great user stories with valuble tips on splitting.