Well, we do have tomes as off hand items (like orbs).
Tightening in spells learned when completing a quest or some objective during a quest would be a fun way to add unique spells to your spellbook.
There are some places in the game where the wizard "drops" scrolls when killed/engaged.
Tho, I mean that as another way to obtain such a spell, having to run the same quests each life would be tedious as well.
No. In the portable hole, spellbooks with non-rare spells. Double click, and you learn all those spells, the price of the spellbook includes both inscription materials and the price of the spells themselves.
Spellbooks as loot: they have a smaller number of spells (preferably those used by the villain in question, but if the devs prefer random, ok), it is again a consumable such as a scroll, when you click on it, you learn the spells of its inside. If you want, you can save spellbooks so when you reincarnate you don't have to spend an hour writing spells.
And that is it. It would be a new type of consumable, and as I say in pnp this exists, learning from your enemies' spellbooks is a classic in D&D wizards.
In any case, the inscription materials should be consolidated into one. And inscription consumes more materials according to the level of the spell. That, along with increasing the stacking to more than 10 materials, of course. And by the way, the writing animation should be reduced by half. It is unnecessarily long.