Per the USPS website, Media Mail IS for books. If you go to http://www.usps.com/business/booksmedia/services.htm you will see that it says:

Media Mail
What you can send:
• Books
• Sound recordings
• Recorded video tapes
• Printed music
• Recorded computer-readable media (such as CDs, DVDs, and diskettes)

I can also tell you that Media Mail has been being used for shipping books for several years by commercial book sellers prior to this new rate increase so it isn't anything new.

Cheers!
Bill