Make achievement books add/stack xp after 24 lvl.
For example:
Character 'X' has 35 lvl and reads lvl 24 book -> all xp from 24 lvl book is added to char 'X' -> char 'X' has 4-something lvl.
Why only after 24 lvl ? Becouse you have to take last perk at 24 lvl, so it will be easier that way and won't mess up anything.
What will it change ? You could buy many low lvl books to buff high lvl char or combine xp from all your alts if needed.
Simple and useful.