Ken Schwaber& Mike Beedle: Agile Software Development with Scrum - The original Scrum book. Good, clear overview of the practices and principles of Scrum. It is somewhat out-of-date now, as Scrum has progressed since the book was written, but it is still a valuable read.
Kent Beck: Extreme Programming Explained: Embrace Change - One of the first Agile books. Focuses mainly (but not solely) on the engineering practices, and supplies good overall context for creating an Agile organization.
Mike Cohn: User Stories Applied - This clear and simple book covers the aspects of writing, estimating, prioritizing and committing to product requirements. Essential reading for Product Owners.
