The rules for spellcasting given in Men & Magic basically are limited to three areas, which appear in the following order:
- SCROLL CREATION: pp. 6-7. Only Magic-Users may create scrolls, and it takes 1 week/spell level and costs 100 GP/spell level/week.
- SPELL USE: p. 19: "The number in each column opposite each applicable character indicates the number of spells of each level that can be used (remembered during any single adventure) by that character. Spells are listed and explained later. A spell used once may not be reused in the same day."
- DUPLICATE SPELLBOOKS: p. 34. Costs 2000 GP for an entire 1st level spellbook, double costs per level. It mentions that, if a caster loses all spellbooks, they must be replaced (presumably before being allowed to cast more spells) and that all this takes is paying the costs. There's no requirement to have an existing copy of a spellbook to create a duplicate.
I think requiring casters to select their spells before the adventure is more fun, but really, the rules leave it up to the GM what the spell limits mean.
What we do know is that there are no per-spell costs given to add a spell to a spellbook. We could use the scroll creation cost directly, lower it (if spells can't be cast from spellbooks as if they were scrolls,) or ignore it completely, which the rules for spellbook duplication imply.
Monsters & Treasure adds two more bits of information about scrolls specifically:
p. 24: "There is a 25% chance that any scroll of spells found will contain those usable by clerics."Note that it doesn't say "usable only by clerics".
p. 32: "All Scrolls are spells for Magic-Users, and regardless of the level of the spell they can be used by any Magic-User capable of reading them. All "Protection" spells can be used by any character who is able to read. Scroll spells are of the 6th level unless necessarily higher, in which case they are of the minimum level necessary to generate such a spell. After reading a spell from a scroll the writing disappears, so the spell is usable one time only!"The first statement is either contradicting the previous comment, or it's telling us that, regardless of whether a scroll is a Magic-User or Cleric spell, a Magic-User (and possibly only a Magic-User) can cast it.
The passage also seems to directly contradict Men & Magic: if only 11th level or higher Wizards can create scrolls, why are most scrolls effectively 6th level?
(See Part II here, for more thoughts on magic in the LBBs.)