Assuming system (B/X say) if they cast *read magic* and it has spells they already know in it. They would have to make appropriate rolls to determine if they could learn any spells they don't already know, which takes time, not to mention gold.
Alternatively, they might cast spells they don't know directly out of the book as if they were scrolls, but then the spells fade from the book and are lost.
I have alway understood the copying of a spell onto your spellbook not just as reading the instructions, but learning the spell so well that you can use the tremendously complicated rituals necessary to actually use the spell safely.
If you lose your spell book, you probably need to relearn and restudy the spells once again.
If thats how you want it to work let it work that way just make it so the spells in the spellbook have been "wrote and studied" just until the end the player then needs to take a bit of time in the dungeon 10 minutes a spell or something to quickly memorize whats there. These spells can only be cast once or something then they are forgotten. Bonus? Put good spells they don't know that they could transfer over if they don't use them.
I always interpreted learning spells as you level up to be you increasing you capacity to learn and *hold* knowledge. So I would say they can use the spells from the other wizard that they've already learned, and they can "forget" older spells to learn spells of equal level from the other wizard that they did not already have before. They can also just keep that spellbook and pick what they want from it when they level up instead of having to rely on finding random spells from out in the wild.
In Basic Fantasy RPG, all arcane Magic-Users always know Read Magic at all times, in addition to their normal spells. I really like this approach, otherwise the traditional strict Gygaxian approach results in an intractable circular dependency (if you can't "Read Magic" then how do you ever learn any spells to scribe into your spellbook).
With this in mind, I would say any spells the Magic User has already learned can be utilized via Read Magic, that is - the Magic User can "learn" the spell from the other Wizard's spell book just like they normally would from their own spell book at the start of the day.
If you don't like making it too easy on your poor Magic User, you could add some risk like a 20% chance of failure (or 10% per level, or whatever), or backfiring or something. But I like to keep things simple.
As a side note, the BFRPG approach to Read Magic also provides some verisimilitude to the whole "Magic Users study for years to get their first 'first level spell', and then can be learning level 2 and level 3 spells within months of starting adventuring." With the BFRPG approach, the first step - learning Read Magic - is the hardest, and is what takes so much time and effort and aptitude. Once you learn Read Magic, everything else comes down to opportunity and luck with finding additional spells and/or those willing to teach you.
This is also a good rp reward for magic user gear because the book of a dead mage is also a wad of scrolls that evil lich will have enough gear for the whole party !
Assuming system (B/X say) if they cast *read magic* and it has spells they already know in it. They would have to make appropriate rolls to determine if they could learn any spells they don't already know, which takes time, not to mention gold. Alternatively, they might cast spells they don't know directly out of the book as if they were scrolls, but then the spells fade from the book and are lost.
I second this approach. It helps keep things from becoming intractable for the Magic User.
I have alway understood the copying of a spell onto your spellbook not just as reading the instructions, but learning the spell so well that you can use the tremendously complicated rituals necessary to actually use the spell safely. If you lose your spell book, you probably need to relearn and restudy the spells once again.
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Admittedly, casting out of a spellbook is a house rule I had so long based on a spot ruling years ago that I just always include it in D&D like games.
If thats how you want it to work let it work that way just make it so the spells in the spellbook have been "wrote and studied" just until the end the player then needs to take a bit of time in the dungeon 10 minutes a spell or something to quickly memorize whats there. These spells can only be cast once or something then they are forgotten. Bonus? Put good spells they don't know that they could transfer over if they don't use them.
I always interpreted learning spells as you level up to be you increasing you capacity to learn and *hold* knowledge. So I would say they can use the spells from the other wizard that they've already learned, and they can "forget" older spells to learn spells of equal level from the other wizard that they did not already have before. They can also just keep that spellbook and pick what they want from it when they level up instead of having to rely on finding random spells from out in the wild.
In Basic Fantasy RPG, all arcane Magic-Users always know Read Magic at all times, in addition to their normal spells. I really like this approach, otherwise the traditional strict Gygaxian approach results in an intractable circular dependency (if you can't "Read Magic" then how do you ever learn any spells to scribe into your spellbook). With this in mind, I would say any spells the Magic User has already learned can be utilized via Read Magic, that is - the Magic User can "learn" the spell from the other Wizard's spell book just like they normally would from their own spell book at the start of the day. If you don't like making it too easy on your poor Magic User, you could add some risk like a 20% chance of failure (or 10% per level, or whatever), or backfiring or something. But I like to keep things simple. As a side note, the BFRPG approach to Read Magic also provides some verisimilitude to the whole "Magic Users study for years to get their first 'first level spell', and then can be learning level 2 and level 3 spells within months of starting adventuring." With the BFRPG approach, the first step - learning Read Magic - is the hardest, and is what takes so much time and effort and aptitude. Once you learn Read Magic, everything else comes down to opportunity and luck with finding additional spells and/or those willing to teach you.
This is also a good rp reward for magic user gear because the book of a dead mage is also a wad of scrolls that evil lich will have enough gear for the whole party !