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?
Does a monster that casts spells as a wizard need to prepare its spells from 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.
1\$\begingroup\$ I can't help but agree, but is there any example of such a creature actually having a spellbook? And should such a creature have the normal number of spells in its spellbook as a wizard of its wizard casting level? (The red ethergaunt is scheduled for use next week, and I need to know if its spellbook should be among its treasure. It's an advanced scout, so it should have one if it needs one, and one of my binds is that it can't, for example, take different feats to, for example, eliminate or partially eliminate its spellbook.) \$\endgroup\$ Jun 10, 2017 at 12:53
\$\begingroup\$ The Encounter Treasure section of the DMG, starting on p51, has the answer to that question. Specifically, in the Building a Treasure sub-section starting on p53, and the Wizards and Treasure sub-section on p54. It is there specifically noted that if you are designing an encounter with a wizard, to subtract the value of the spellbook from the average treasure value of said encounter before rolling up the rest of the treasure results. The method of calculating spellbook values are found on p179 of the PHB. Ah, should this be a separate question, perhaps? ^^ Indirect, but works? \$\endgroup\$ Jun 11, 2017 at 23:03
2\$\begingroup\$ This of course is different from whether or not a scout-type professional would or should actually carry a spellbook into a possible combat situation. There are also methods of disguising spellbooks via alternate styles of spell storage: carvings, weavings, knots, pictures, tiles, tattoos, and so forth. There is a general assumption in the game that wizards don't put all their eggs (spells or books) in one basket, having backup spellbooks, scrolls, and some even having different sets of spellbooks for different situations. \$\endgroup\$ Jun 11, 2017 at 23:10
\$\begingroup\$ As for an example, if you set aside every example NPC wizard in any of the other books, magazines, modules, or adventures... I can't recall offhand if any of the 5 monster manuals (plus a few more settings specific monster manuals) have spellbooks listed in the treasure of a given monster. I suppose we could check the treasure tables in the DMG.... \$\endgroup\$ Jun 11, 2017 at 23:12
\$\begingroup\$ Huh. That's interesting — and I can totally see adding to the current answer clarification on the comment's second point (i.e. probably the normal number of spells in the spellbook plus however many the DM wants to add, subtracting added spells' values from the monster's treasure), but I just went through the MMs, and I don't think anything besides the drider and 'gaunts has natural wizard casting. If another creature somewhere has a spellbook and natural wizard casting to confirm the answer's theory, that'd be perfect. But don't work too hard—you already won this.
:-)\$\endgroup\$ Jun 11, 2017 at 23:29