I have a weird, old rack that screws into braze-ons near the seat post, but then uses clamps to attach to the rear triangle, above the wheel. It's rated for 55 lbs; if you could find a similar rack that also clamps to the seat post, like clip on fenders, it should have a decent weight capacity.
How much does everything you want to carry weigh? Books are heavy, but how many do you bring with you at a time? I've been carrying a single book and a few other things on my rack, in a kayaking dry bag for the rain.