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 SPELLS KNOWN
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.