to treat it as emergency spell prep, as if you were memorizing the spell right there in the dungeon, which means it requires extra time and is basically ritual magic. Other people, though, have suggested treating spellbooks as scrolls, with no difference in casting.
But here is something to consider: can a Magic-User read a scroll while wearing armor? We're assuming the strictest interpretation of the spells and armor rules in the LBBs, for the moment, that M-Us can't cast spells while wearing armor. But can they use a scroll? If yes (the default assumption,) can they cast from a spellbook while wearing armor? Because if they can, that makes armor restrictions on general spell use seem pretty arbitrary.
My assumption is that there is a difference between a spell on a scroll and one in a book. The one in a book is a set of directions for casting a spell; the one on a scroll is a spell that has already been cast, with magic words inscribed as a trigger to activate the spell. So, scrolls can be read without problem.