Book six, in sticking with the agile theme, is User Stories Applied: For Agile Software Development by Mike Cohn (ISBN 0321205685).
There are in general two major forms of requirements documents, Use Cases and User Stories. Use Cases have been around for a long time, and if you work in a larger company you have probably been exposed to them a lot. User Stories on the other hand are a newer product that is mostly used in agile development. The idea behind a user story is to create less detailed specifications and from the user's point of view.
This book will walk you through the theories behind User Stories and show you the proper way to create them.
This book is available on Safari Bookshelf.
(This post is part of an ongoing series to build your technical library.)