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.
What's interesting here is that, although it refers to remembering spells during an adventure, it doesn't refer to preparing them in advance at all; it doesn't even say that the spells must be selected in advance. It does, however, say that each spell may only be used once. Since there's no clarification of this as "A prepared spell used once may not be reused in the same day", this suggests that a Magic-User can't cast multiple Fireballs in a single day. This would explain why scrolls and wands exist; they are ways of circumventing this limitation.
There's also a specification that the numbers represent how many spells can be used per adventure, not per day, which means that Magic-Users were originally not intended to renew their spells on a daily basis. However, one consequence of this interpretation is that, if an adventure lasts longer than a day, an M-U can in fact cast Fireball more than once; the M-U just can't cast it more than once a day.
That's clearly not the way I or other people play it, but I'm going to make it the default interpretation, with the more common interpretation listed as an alternative.
This leads to the interpretation of what spell books are. They are mentioned on page 33:
Characters who employ spells are assumed to acquire books containing the spells they can use, one book for each level. If a duplicate set of such books is desired, the cost will be the same as the initial investment for research as listed above, i.e. 2,000, 4,000, 8,000, etc. Loss of these books will require replacement at the above expense.The text clearly applies to both Magic-Users and Clerics. Interestingly, there's no mention of individual spells being contained in the spell books; a Magic-User can basically just spend money and do 1 week of research per spell level to replace an entire spell book. Spell books can contain notes on unique spells, perhaps, which would make stealing a dungeon-dwelling sorcerer's books worthwhile. And presumably a spell-caster can't renew spells without spell books, even though this is not mentioned. Spell books only seem to be a "money pit", by the books.
But spell books can't be interpreted without Read Magic, right? Here's what page 22 says about the spell:
The means by which the incantations on an item or scroll are read. Without such a spell or similar device magic is unintelligible to even a Magic-User. The spell is of short duration (one or two readings being the usual limit).This seems to restrict the intention of Read Magic to the use of scrolls and magic items. An M-U who doesn't have Read Magic can't use a scroll or wand found on an adventure.