As stated in the title. A solution is to exclude the spellbook from the logic used.

It should only be hooking the spellbook in a secure fashion (and some other secure frames that way too) but I'll see if I can track down what's causing the taint.

Its not the hook, its adding the SpellBook to the UISpecialFrames list that's the issue.

Blizzard altered their "Hide" actions to no longer be a taint barrier, so if the SpellBook is in the list it then gets tainted by other addons whose frames are in the list.