The captured spell book is a nebulous construct in BX and OSE. The item definitively exists in the game’s ruleset, yet comes with almost no explanation.
In OSE, this is all we get:
[Captured Spell Books: Without the use of magic (e.g. read magic), a spell book can only be read by its owner.]
That’s it. We have to infer everything else. And it’s not as though the captured spell book is a rarity; low-level MU and Elf PCs keel over regularly, leaving a pile of gear and a spell book on the floor of the dungeon between players who are left to wonder, “Well, what can we do with it?”
And sure, every referee probably has their own custom rules, but this is a RAW blog, so let’s see if we can piece together an objectively righteous baseline understanding.
1. You can read it.
As implied by the description, each spell book is written in a way that is completely unique to its author and owner. Casting Read Magic permanently enables the caster to read the text of that spell book.
The spell book definition and rules then tells us:
“A character’s spell book contains exactly the number of spells that the character is capable of memorizing (as determined by the character’s class and level).”
Some readers take this to mean, “You can only use your own spell book because it contains only the spells you are able to cast”. This leads to making home-brew rulings like, “Captured spell books can only be used to reduce the time and cost of magical research by 50%.”
Which is lovely, but it’s not RAW. It’s a whole new rule with new conditions.
We don’t need home brew to resolve this, though. The ink on the page tells us everything we need to know:
2. You can’t own it.
We’ve established that authorship is ownership. Your spell book contains exactly the number of spells that your character (its author and owner) can memorize. The captured spell book contains exactly the number of spells that its author could memorize. So you can carry someone else’s book, and you can translate and read someone else’s book… you just can’t own their book. It’s always theirs. Which brings us neatly to the next point:
3. You can’t write in it.
If you were to record additional spells in a captured spell book, you would become an author and therefore its owner. That would violate the first rule of spell book ownership: your book contains exactly the number of spells your character is capable of memorizing. Authoring (and thereby owning) a second book of spells would mean your book(s) contain more spells than you are capable of memorizing.
4. You can memorize from it.
If you can read it, you can memorize it. If you can memorize it, you can cast it. And this is where the RAW text champions this argument. Because the rules do not say that your spell book contains the exact spells you are capable of memorizing… the rules say it contains exactly the number of spells you are capable of memorizing. The limitation is on number only.
If you’re a 3rd level Elf, you can memorize exactly 3 spells, so your book can contain exactly 3 spells.
The spell book of your now-dead 3rd level MU companion also contains 3 spells, and can only ever contain 3 spells; exactly the number its owner was capable of memorizing.
The books are limited by their authors.
The authors are not limited by their books.
You can memorize any 3 spells that you can read.
5. You can’t copy from it for free
The game tells us there are only two ways to add spells to your own book: Magical research at 1000gp/2w per spell level, or mentorship. If we take that at face value, there’s no shortcut to writing new spells into your book. It costs time and money to copy it yourself, or you get help from your mentor.
I know copying a spell from a captured book might seem wastefully redundant, but to reduce the encumbrance of adventuring with a set of encyclopedias strapped to one’s back, the caster might find it pragmatically expeditious to record the “best of” in their own book.
6. Value and narrative
In-world, magical versatility would be a reasonable thing to conflate with power and esteem among wizards. More spells, more power, more important. But carrying a library around would be problematic, and hiring a retainer to carry the extra weight would drain the party’s XP. So what’s a wizard to do?
Well, it stands to reason that this is why wizards build stronghold towers; to secure their libraries and to dwell where they have instant access to the full catalog of their magic. They adventure early to accumulate spell books and magic items and bring them back to their masters, establish a tower, and then take on apprentices (L1 Mediums) to send them out and reclaim captured spell books on their behalf.
And I think that’s a key Vancian dynamic that’s written between the lines of the BX and classic OSE game: The tenuous relationship between master and mentee casters. And I believe this is (sadly) often hand-waved, because it’s such a beautifully rich aspect of the world’s implied culture:
Each level 1 MU is sent adventuring with the same mission: Bring back more magic to the master—books, items for research, etc— and in exchange, the master will mentor you and teach you more spells. The more spells you bring back, the more spells the master can help you to learn. It’s mutually beneficial… until it isn’t…
Because at some point, it inevitably becomes treacherous. The master with a powerful pupil feels threatened, because the question isn’t, “Are they holding out on me” but “How much!?”. And the pupil who approaches their master’s power might start to covet the very library they helped to build. It’s no surprise that Wizards shut themselves in their towers, trusting no one at the gates for fear of assassins, trapping their lairs against intruders and thieves, and generally becoming secretive hoarders as rumors spread among the townsfolk… and so on.
Seen from that perspective, the captured spell book might very well be at the crux of the entire genre.
As always, food for thought at your table.
Thanks for reading!
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