So I think this definitely should be about how many very rates you want in the game. It nay be significantly influenced by Catwalks algorithm for guaranteed spells based on books.
But if we go under the assumption that we leave the spells per book as it is, then we shouldn't want to have a lot more very rares in the game.
Ideally, what I would love to have, but probably can't because its a new mechanic, is scrolls. Instead of finding spells, you find a scroll of a spell. You can either copy the scroll out into your spellbook (if its a rarity and realm your books grant access to) in which case you learn it just as finding s spell currently does, or you can use the scroll to cast the spell once - if used to cast the spell, then you don't expend the mana required, the scroll does so for you. However you must still provide the casting skill - so if you had 50 mana and 30 casting skill, and had a scroll of phantom beast, you wouldn't be able to use the scroll in combat until you had 35 casting skill. Similarly, if you had a scoll of summon naga it would take you a little over 3 turns to cast it, but (assuming no mama income) you would still have your 50 mana.
This system would specifically allow you to find spells you already know; and spells of realms you had no books in. If you do have any books or retorts that would affect casting a spell (such as conjuror, or 8 sorcery books in the example above) they WOULD apply to scrolls, by doing things like reducing required casting skill.
A scroll would only be used up once successfully cast. If you started to cast summon naga, and changed spells or canceled it after 1 turn, you would not lose the scroll.
If doing this system, I would increase the occurrence of found scrolls compared to the current amount of found spells. However, I would also increase the likelihood of finding common and uncommon spells, while leaving rare and very rare spells the same.
Super ideally, you could steal scrolls from wizards when you banish them. (1-4 per banish, randomly chosen from scrolls the banished wizard owns).
Additionally, when banishing a wizard, instead of stealing spells he knows, you would get to inscribe 1-4 scrolls - these would have to be spells he knows, of realms and rarity you have access to via your own spellbooks. 1 of these scrolls (if possible) would be of the highest rarity spell he knows that is also a spell you don't know. Any others would be randomly picked from spells he knows, but would be most likely to be of the rarity matching the highest rarity spell you have researched (say, 40% to match your highest rarity, 30% to be one rarity lower, 20% to be one rarity higher, and 10% to be the fourth rarity. In the case the case that your highest rarity is either common or very common, then 40% to be same, 30% to be 1 difference in rarity, 20% to be 2 different, 10% to be 3 different).
I'm fully aware this is probably impossible to implement as it would require a new tab in your spell book (both combat and overland) but whatever, I had to suggest it.
I'd also probably give a penalty to casting skill if you don't gave access to the right rarity or realm of a scroll. Like it takes 10% more to cast a spell of one rarity higher than you normally can in a realm (common if 0 books, very rare if 3 books); 25% more if two rarities beyond your access (uncommon if 0 books, very rare if 2 books), 50% more if 3 rarities (rare if 0 books, very rare if 1 book), and 100% more if very rare with 0 books.
I'll keep thinking more about more realistic suggestions.
But if we go under the assumption that we leave the spells per book as it is, then we shouldn't want to have a lot more very rares in the game.
Ideally, what I would love to have, but probably can't because its a new mechanic, is scrolls. Instead of finding spells, you find a scroll of a spell. You can either copy the scroll out into your spellbook (if its a rarity and realm your books grant access to) in which case you learn it just as finding s spell currently does, or you can use the scroll to cast the spell once - if used to cast the spell, then you don't expend the mana required, the scroll does so for you. However you must still provide the casting skill - so if you had 50 mana and 30 casting skill, and had a scroll of phantom beast, you wouldn't be able to use the scroll in combat until you had 35 casting skill. Similarly, if you had a scoll of summon naga it would take you a little over 3 turns to cast it, but (assuming no mama income) you would still have your 50 mana.
This system would specifically allow you to find spells you already know; and spells of realms you had no books in. If you do have any books or retorts that would affect casting a spell (such as conjuror, or 8 sorcery books in the example above) they WOULD apply to scrolls, by doing things like reducing required casting skill.
A scroll would only be used up once successfully cast. If you started to cast summon naga, and changed spells or canceled it after 1 turn, you would not lose the scroll.
If doing this system, I would increase the occurrence of found scrolls compared to the current amount of found spells. However, I would also increase the likelihood of finding common and uncommon spells, while leaving rare and very rare spells the same.
Super ideally, you could steal scrolls from wizards when you banish them. (1-4 per banish, randomly chosen from scrolls the banished wizard owns).
Additionally, when banishing a wizard, instead of stealing spells he knows, you would get to inscribe 1-4 scrolls - these would have to be spells he knows, of realms and rarity you have access to via your own spellbooks. 1 of these scrolls (if possible) would be of the highest rarity spell he knows that is also a spell you don't know. Any others would be randomly picked from spells he knows, but would be most likely to be of the rarity matching the highest rarity spell you have researched (say, 40% to match your highest rarity, 30% to be one rarity lower, 20% to be one rarity higher, and 10% to be the fourth rarity. In the case the case that your highest rarity is either common or very common, then 40% to be same, 30% to be 1 difference in rarity, 20% to be 2 different, 10% to be 3 different).
I'm fully aware this is probably impossible to implement as it would require a new tab in your spell book (both combat and overland) but whatever, I had to suggest it.
I'd also probably give a penalty to casting skill if you don't gave access to the right rarity or realm of a scroll. Like it takes 10% more to cast a spell of one rarity higher than you normally can in a realm (common if 0 books, very rare if 3 books); 25% more if two rarities beyond your access (uncommon if 0 books, very rare if 2 books), 50% more if 3 rarities (rare if 0 books, very rare if 1 book), and 100% more if very rare with 0 books.
I'll keep thinking more about more realistic suggestions.