Manage spellbook and navigation
After some time I located a bit of code that did not execute the same way on my development machine..You should now be able to switch between which spellbook that is active regardless what page you are visiting.
But keep in mind.
If you visit the spell-page or a certain spell that is included in your current book and switch to a new one, the add / remove button for that spell will not change / updated to inflict the changes (perhaps in the future). But of course the spell can only be added once in a book.