MediaMail postal rate can be used to send books (of at least 8 pages), film, printed music, printed testing materials, sound recordings, video recordings and educational charts. 

Media mail is, in the words of USPS, "Perfect for sending used books".

The Media Mail regulations are shown in the Domestic Mail Manual on the USPS site at the following page:

http://pe.usps.com/text/dmm300/173.htm#1113509

(Scroll down to 4.0, Content Standards for Media Mail > Qualified items)

Sending anything other than that described in the regulations by Media Mail postal rate is illegal. 

Media Mail packages are subject to random postal inspection and if a package is found to contain unacceptable items/materials, the difference between First Class rate (or Priority rate if the package weighs over 13 ounces) and Media Mail postage will be assessed.  Packages that are found to violate Media Mail postal rate may be returned to the sender, OR they may be delivered postage-due to the destination address.

Related links:

May I include anything with my book?
How much does it cost to mail a book?
Do I have to mail from a post office?