have two books on my shelf that are college textbooks, library discards, so they may not meet the conditions of PBS.

Textbooks and library discards are postable as long as they meet the conditions set forth by PBS rules.

Should I remove them or should they remain on my shelf since they are worth a moderate amount and in demand for current courses.

The question you should ask yourself is How charitable are you?

 Do you want to give away an in-demand textbook and pay for shipping on top of that or would you rather sell it and make a little cash?  I sell my in-demand texts on half.com or Amazon. Suckers  Really generous people give away expensive texts books and pay 5 - 8 bucks on shipping.

Unless your book weighs 8 pounds it won't cost you 5 bucks!  Here's the media rates.  Funky format but this is it.  lol