About Arcane Spells Known

A few of you have raised great points about the meta-game explanation for how magic works in ACKS. I re-wrote the Arcane Spells Known section to better explain the metaphysics of arcane magic.
In the process of doing so, it occured to me that ACKS probably needed different game mechanics for what happens when you LOSE a spellbook, so I wrote some new material there. I think the final result makes great sense within the context of the game world, and is highly playable. Please let me know if you agree!

Arcane spellcasters know only a limited number of spells. Knowing an arcane spell is far different than merely possessing a copy of its formula. For an arcane spellcaster to know how to cast a spell he must keep track of complex astrological movements and star signs that are constantly changing; he must daily appease various ghosts and spirits that power certain dweomers; he must remember and obey special taboos that each spell dictates. All of these strictures, and they are many, can vary with the season, the lunar cycle, the caster’s location, and more. Knowing a spell is thus an ongoing effort, like maintaining a friendship or a muscle. Only the most intelligent and learned arcane spellcasters can know more than a few spells at a time.
An arcane spellcaster can know a maximum number of spells equal to the number and level of spells listed for his level, modified by his Intelligence bonus. For instance, a 3rd level mage is able to cast 2 1st and 1 2nd level spell per day. If he has 16 INT (+2 modifier) he can know up to 4 1st level and 3 2nd level spells.
An arcane spellcaster records his known spells in his spell books. Spell books are not encyclopedias of spell formulae; they are more like journals, in which each day the caster reviews and re-calculates what he must remember about each spell he knows. Spell books are esoteric and personal in nature, so they are legible only to the spellcaster who wrote the book, or through the use of the first-level spell read magic.
Arcane spellcasters can learn new spells in a few different ways. All mages and elven spellswords are assumed to be members of the local mages’ guild, or apprenticed to a higher level NPC. When they gain a level of experience, they may return to their masters and be out of play for one game week while they are learning their new spells. Their masters will teach them spells equal to the number and level of spells the caster can use in a single day. Characters of 9th level or above do not have masters to teach them spells, so they must find or research them. When a master is not available, mages and elven spellswords depend entirely on finding spell scrolls to add to a spell book, finding other spell books with new spells in them, or conducting spell research.
If a new spell is found on a scroll, or another arcane spellcaster’s spell book, it may be added to the arcane spellcaster’s spell book, if the character can still learn new spells of that level. If the spell is of too high level to be cast, it cannot be put into the book, but it may be saved to be put into a book in the future. It takes one week of study to scribe a spell into the character’s spell book. Scribing a spell from a scroll uses it up in the process, but copying spells from one spell book to another does not erase spells from the book.
Sometimes an arcane spellcaster’s spell book will either be lost or destroyed. Each week he goes without access to his spell book, an arcane spellcaster loses the knowledge of one spell level, until eventually he knows none at all. An arcane spellcaster can rewrite the spells through research and memory at a cost of 1 week of game time and 1,000 gp for each spell level. For instance, if two first level spells and one 2nd level spell are replaced, it will take 4 weeks and 4,000 gp. This activity requires complete concentration, and a character doing this work may not engage in any other activity for the time required.
An arcane spellcaster can follow the same procedure to permanently replace one spell in his spell book with another. For instance, Quintus is a 1st level mage with INT 16. He is eligible to have 3 1st level spells in his spell book. Over time, he has recorded read magic, shield, and sleep into his book. He finds a grimoire holding magic missile and decides to replace shield with his new find. (That is, Quintus decides to stop actively monitoring the various stars, spirits, or taboos associated with shield so he can instead pay attention to those associated with magic missile). This costs 1 week of game time and 1,000gp. In game terms, he now knows read magic, magic missile, and sleep, but no longer knows shield and therefore cannot cast it.

Works for me. I like it. If I squint really hard, I can still see it as being quite Vancian, but with a nice hyperborean/melnibonean twist.

I love making the theory of stars and spirits concrete, it definitely lays the groundwork for some of the cursed items, spells, etc. we’ve talked about at the Mule when discussing this topic.

Are mages to keep track of the spells stored in all the grimoires they possess? I ask because the cost stated above is half the cost of researching a spell from scratch, and this would encourage a mage to begin hoarding arcane knowledge early in their career. For example, a mage rewrites shield with magic missile, but in a year he wishes to relearn shield. Instead of researching shield from scratch using the spell research system, he simply begins to update his old practices pertaining to shield.
This would make the loss of a magical sanctum especially harmful. The mage would lose all his formulae, library investments, workshop investments, and spell grimoires.

Artus, you have grasped the intent of the system exactly! Mages hoard everything they can because it’s far easier to learn from a spellbook than from research.

Love it. The “traditional” D&D spellbook becomes the mage’s scroll/grimoire library, so the flexibility is still there if you need to prepare for a particular type of adventure, it’s just shifted in time scale.
I like the image of a mage’s morning routine being a combination of chanting a few verses of an old prayer to a forgotten god, marking the position of a star, and burning three particular herbs because it’s Wednesday. I think that shifts the image of the mage away from the overly-scholarly bookworm (with apologies to Agrippa) and closer to someone you’d find kicking in a dungeon door.
Lots of room for flavor if a GM wanted to make some of those stars/spirits/taboos explicit, and it provides a nice loose-but-consistent framework for things like spells that only work during the full moon or cultists waiting until the stars are right.
I wonder if “spells active” might be clearer terminology than “spells known” (or something along that line).

I like the idea, but I think there is a disconnect between the first part of the explanation and the second part of it.
I think that the idea of it’s not spell known but spell “fed” is great and must seep deeper in the rules.
From what you have written I got the following general idea, a spell formula is the starting point of readying a spell, to have a spell in the spellbook a mage must do some sort of magical activity (a mix of an horoscope, solving an equation, all modified by where the wizard is and who he is…) so the spellbook is the solution to that particular problem and must be slightly corrected day by day… to lose the spellbook means that the mage must start again from the general formula…
that said I like it, this means that the mage can have huge collection of formulas but still need a lot of time to “prepare” the spellbook with his spell