As Sir Whiskers said spells doesnt count toward book slots. If you sell all scrolls in some castle you`ll be able to buy anything you want in shops learn them, and later return to that castle and buy scrolls you sold if you want them.
About non cheating way to increase scroll slots there are items you can equip buy some scroll, and then remove to use your usual items : wisdom_scull (+20 scrolls) , scroll_bag (+8 scrolls) , set_shaman (+5 scrolls) , set_inquisitor (+5 scrolls). SO if you find any of this items (sets) and are near shop with interesting scroll, then simply open hero screen, place those items in hero item slots, buy scrolls you want and place back your original equipment. No cheating, no problems.
Good luck
