No Room in Spell Book
 

I often get this message when I want to buy a spell. What makes room in your spell book? (I'm a paladin.)

Also: do you have to feed Sleen more than once? He's been very cooperative since I gave him some venemous spiders, but will he need more later?

Hover your mouse over the spell book, in hero screen, if should show, how many space are left. Space in your spell book are most likely occupied by the scrolls, use, sell, or write them as spells :).