Current Behavior: In BG2 you may have more spells memorized per level than can be shoved into 1 page (>12).
Expected Behavior: option 1: re-design the spell book in order 1 page to hold 20 spells per level. option 2: add "slider" option 3: cap maximum memorized spell amount at 12 (current amount of available "windows").
P.S. Actually I am not not sure is this a bug or just design flaw. Thus I have also created similar topic in Feature Requests.