One of the things I always ignored about B/X was the strict rules on wizards' spellbooks. Probably because my first gaming group mixed B/X and 1e liberally, I hated the idea that a wizard couldn't find lots of spells hidden in dungeons and add them to their library. By B/X rules as written, you can find a million spells and still only be able to write into your own book the exact number of spells you can cast per day, per your level.
Of course this is a game and they were intentionally trying to make it simple. And it works. It's clean and fast and easy. And if I allow myself to just accept this as the rules and then think about how it can be justified in the fiction I can come up with some cool ideas, I think.
Every Spell is Unique
Magic spells are like living entities. They exist in only a single place in a single book. The magic-user who has Fireball in their spellbook is definitely going to be the Flame Wizard or something like that. In this version of the fiction wizards would necessarily be rare simply because finding spells is god damn hard.
So the magic-user who wants the power to create light must find the wizard who is the master of the Light spell and somehow convince them to give the spell over. Maybe they barter powerful magic items or weird arcane paraphernalia. Or maybe one wizard slays the other in magical combat. Either way, perhaps the rules need tweaked a little bit to allow the acquiring wizard to transfer the Light spell from the other wizard's book into their own. Or, since we're talking fantasy here, the Light spell, knowing it has a new master, transfers itself.
This idea makes the rules on page X51 (page 59 of the Old School Essentials Rules Tome) much more attractive to the player. If I can't find a new spell to cast I can create one! This also gives the players and DM more incentive to have weird things in the campaign. For example, to cast Light, you don't have to have anything. You just say the words and make a gesture. But to research a new spell you need to find rare materials. Now's your chance to actually have eye of newt be an important thing in your game. Do you have eye of newt just lying around? You've been murder-hoboing around the dungeons for 3 levels. When have you had time to set up a proper workshop?
This type of campaign could be a lot of fun for the magic-user and DM. But it feels like it might be the party wizard who is dictating campaign goals. The fighter and halfing are just looking to spend some coin on grog and go bust goblin heads.
NOTE: I vaguely remembered a blog post from back in the glory days of he OSR where someone pontificated this same idea, more or less. And it is here on Ode to Black Dougal.
Rules as Written, Buddy
So in this fiction spells are not unique entities, they are just rare and hard to come by. This is actually
The biggest difference here is that Read Magic is an even more important spell than anything else. It's true even in the concept outlined above, but with that idea you could have the house rule that allows spells to be transferred from one book to another. Here you don't have that option. You must be taught or you must do research. I suspect a good DM would require the use of Read Magic quite a bit in this kind of campaign.
As an aside, I never ever ever paid attention to the rule that you have to use Read Magic to cast from a scroll. Or even a book. My rule was always that anyone can read a scroll and any wizard can read a spell from a book. But in either case, reading the spell makes it vanish (or explode, disintegrate, slither off the page, etc.). In the hardcore RAW campaign, I think you'd have to ensure that your party's wizard has Read Magic as their first spell or else make damn sure it becomes available quickly or a magic item bestowing it becomes available.
It Is the Way
In this fiction the wizard is obeying ancient laws and customs. This might be because of some grand all-powerful order of wizards or because of the hand of the gods. In either case, the magic-user will only transcribe the exact number and levels of spells they are able to cast per their level. Not because they can't do otherwise, but because doing otherwise is unthinkable, like kicking a puppy.
You can take this one a step further and say that the wizard actually has levels in the fiction, not just in the game. So those level titles become concrete in-fiction devices. You're on the deepest level of the dungeon and you just hit 20,000 XP? You'll have to wait until you can make it back to the High Yellow Tower and demonstrate your skills in order for the Lofty Sorceress to initiate you into the ranks of the Enchanters.
By this style of play you open things up to a ton of potential fun. Because you know that one player is going to break the rules. They're going to thumb their nose at the Order like a witch slinging Avada Kedavra all over the halls of Hogwarts. Let them! What are the consequences going to be? Will the other wizards come calling? Will the gods themselves show up? Will the magic-user's body be twisted and ruined as if cursed each time they cast more spells than they are supposed to?