Some driders (Monster Manual 89-90) can "cast spells as 6th-level… wizards," and a red ethergaunt (Monster Manual II 66-7) and other ethergaunts "can cast arcane spells as a… wizard." Must these creatures and others like them that cast spells as wizards prepare their spells from spellbooks? Or, for example, does the ability to cast spells as a wizard only impact the details of such creatures' casting—like they risk arcane spell failure if they wear armor and they can prepare spells with metamagic feats applied—, yet the creatures can prepare any wizard spell without consulting a spellbook?
Yes, they follow the same rules...
... except as follows:
(From the core Monster Manual glossary, emphasis added):
Spells: Sometimes a creature can cast arcane or divine spells just as a member of a spellcasting class can (and can activate magic items accordingly). Such creatures are subject to the same spellcasting rules that characters are, except as follows.
A spellcasting creature that lacks hands or arms can provide any somatic component a spell might require by moving its body. Such a creature also does need material components for its spells. The creature can cast the spell by either touching the required component (but not if the component is in another creature’s possession) or having the required component on its person. Sometimes spellcasting creatures utilize the Eschew Materials feat to avoid fussing with noncostly components.
A spellcasting creature is not actually a member of a class unless its entry says so, and it does not gain any class abilities. For example, a creature that casts arcane spells as a sorcerer cannot acquire a familiar. A creature with access to cleric spells must prepare them in the normal manner and receives domain spells if noted, but it does not receive domain granted powers unless it has at least one level in the cleric class.
In summary, as these listed exceptions do not exempt the creature from needing a spellbook (for wizard types), then yes, they do require a spellbook.