I didn't make a big deal of it last time, but in the section on the costs of spellbooks (Men & Magic, p. 34) it states simply "Characters who employ spells are assumed to acquire books containing the spells they can use, one book for each level." As many have pointed out, there is no exception given for Clerics; this comes along in Greyhawk. Also, the rules are not clear as to whether characters must pay for their first "book of second level spells" and so on.
Immediately before the spellbook rules are the magic research rules, and the rules plainly state: "Both Magic-Users and Clerics may attempt to expand on the spells listed (as applicable by class)." The "as applicable by class" might be an indication that a Magic-User cannot research a healing spell usable by Magic-Users, and a Cleric cannot research a Magic-User spell. Or, perhaps, it only indicates that the spells they create can only be added to their respective spell lists and will only be usable by their class; if a Magic-User finds a Cleric spellbook, it might count as research materials, but the M-U can't just copy the spells therein; they must research.
I previously investigated Read Magic, but I should point out again, for the purposes of this series, that the Read Magic spell only states that it is the "means by which the incantations on an item or scroll are read." It does not mention spellbooks, which leaves open the question of whether spellbooks really contain spells, in the same sense that scrolls do. Also, since magic is specified as being unreadable without this spell, and Clerics do not have Read Magic, this is possible further support for the idea that Cleric scrolls are meant to be used by Magic-Users.
What I think this all means is that, although Gary and Dave had more detailed rules in place, even before the publication of the LBBs, the magic system in the LBBs is intentionally sparse and missing many of the elements we later assume to be distinctive. I think the by-the-book magic system is this:
- Spellcasters of any sort have books. If they lose their books, they can't choose spells on their next adventure.
- They do not, however, forget how to cast their spells. The books just enable spellcasting in some way, and do not contain spells except maybe as notes on how to use the tools in the books.
- Spellcasters are limited to a certain number of spells every adventure, based on level. The backstory on why is up to the GM or group.
- Scrolls are a way around this limit, but are expensive. They also allow casters to cast spells they couldn't otherwise use.
- Read Magic allows you to use a scroll. You can't normally use scrolls without it.