i have a set of Carousel Design Works bags
www.CarouselDesignWorks.com
and I've used them for over a year
done a few tours, etc...

so the deal with a seat bag, or any compression bag
is that really, they are designed to accommodate things like clothing
some soft foods, etc...

especially being set up high, ideally, the load would be light weight stuff.

so something like 8-9lbs of books
uhh... i'd think of something different.
the hard edges of books, etc...
its just going to get all bent up
the bag is going to take a bunch of abuse from those sharp edges, etc...

a pannier with a rack would be better.