I mailed books at media mail rate for the first time today (have just been doing 1st class or parcel post) and I hope I did it right.  I don't use the PBS wrapping as I don't have a printer at home, so I hand addressed them, weighed them on the scale (13.2 oz, darn it) and bought 2.47 stamp for that one and 2.81 for the hardback in a box (2 lbs and some oz).  I marked both of them with a black sharpie "media mail".  The hardback was in a cardboard box wrapped in a plastic bag.  Will I get in trouble because of the plastic bag?  Seems like they are really picky about those kinds of things.  Thanks for any advice!

Sounds as if you did fine to me.  

Rachel, under 16 ounces is $2.13 to mail media rate.  The 13 ounce mark is when you have to start using trackable postage, or if using stamps, hand the package in at the post office.