[Fatt_Shade]

You can go in some castle , click on spell book and right click on scrolls you have to many. New console will open for selling those scrolls, so you can clear them from your spell book and make room for those you want to buy and learn.
After this, all this scrolls will stay in that castle and you can buy them later if in case you need them.

One more thing : each class get 1 more scroll slot in spell book, after certain lvlup (warrior get 1 more every 6 lvls, paladin every 5, and mage every 4 lvls). You can change that modding hero.txt file in this part (this is example for mage)

 // ďđîęŕ÷ęŕ
    level_up {
        leadership=10 //40,30
        book=1,4 // đŕńňĺň íŕ 1 ęŕćäűé 4 óđîâĺíü

You see 1,4 part. This give mage 1 more scroll slot in spell book, every 4 lvls. Change it to 1,1, and you`ll get 1 more scroll each new hero lvl.

Other way is harder, it need to start new game, and change hero.txt file in other part (starting scrolls in book) like here (again mage example):

     start {
        leadership=100
        attack=0
        defense=0
        intellect=3
        mana=20
        rage=10
        gold=1000
        rune_might=1
        rune_mind=1
        rune_magic=6
        crystals=20
        book=10

Change book=10 , to any number you want book=1000 will give you 1000 scroll places in book on start of new game.
If you can find hero.txt file open it with notpad/wordpad and make this changes save file and game will take those changes after you play game next time.
Good luck