Agile Software Requirements: Lean Requirements Practices for Teams, Programs, and the Enterprise by Dean Leffingwell provides a detailed study of how to gather and manage requirements in an agile enterprise. It filled in a lot of blanks for me.
An agile enterprise is not easy. An enterprise may do agile but not be agile. That is, the enterprise may have teams that create user stories and have regular standups. But there is much, much more. This book provides for how to be agile.
The book does a good job of defining requirements at three different levels in an organization: portfolio, program and team. Themes to epics to features to stories to tasks. The book is clear and concise. I really liked the use of meta models to show hierarchy and relationships of the agile artifacts.