David, you might start with my latest book, Software Requirements Essentials. My co-author, Candase Hokanson, has a lot of agile experience, so we made sure the 20 practices we present are applicable to all types of projects.

For more depth on requirements, Software Requirements, 3rd Edition is very comprehensive, also with guidance for application to agile projects.

Software Development Pearls describes 60 lessons I've learned about requirements, design, project management, quality, culture and teamwork, and process improvement. I think you'll find that these lessons apply as well to agile projects as to traditional or hybrid projects.

Agilists have to be careful not to discard everything that has been learned in the past, but rather to retain the valuable techniques and adapt them as appropriate (usually by slicing into smaller chunks) to the nature of their projects, culture, team structure, goals, and constraints.