Extreme Programming Explained - Embrace Change by Kent Beck with Cynthia Andres. You really can't understand Agile Software Development without reading this book. This is where it started. I prefer the 1st edition. It is a little more in your face, and it has a picture of the back of my head.
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.
User Story Mapping: Discover the Whole Story, Build the Right Product This is Jeff Patton's great book on how to understand your product.
Working Effectively with Legacy Code by Michael Feathers. The go to book in how to rehabilitate big balls of mud.
Large Scale Scrum: More with LeSS by Craig Larman and Bas Vodde. There is an awful lot of bad information about scaling out there. This is the place to find out what you should actually do.
Writing Effective Use Cases by Alistair Cockburn. There are times when Use Cases are the right thing and Alistair can tell you how to write them, plus very good ideas that will help whether you are using Use Cases or not.
Ivar Jacobson's article on User Case 2.0 explains how they are not quite Use Cases and not quite User Stories. You my find them to be exactly what you need.
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.
Liftoff, Second Edition by Diana Larsen and Ainsley Nies. THE guide to all the things you need to do before your first Sprint.
Waltzing With Bears, Managing Risk On SoftWare Projects by Tom DeMarco and Tim Lister. A valuble guide to landmines that are on our path to successful software development.
Fifty Quick Ideas to Improve Your User Stories by Gojko Adzic and David Evans. A great resource for all things User Story. It has a great section on splitting.
www.retromat.org A great source for doing effective Retrospectives.
The HBR article that started all of this, The New New Product Development Game.