That's a good point. And, in particular, I like the idea that there are some unique spells that characters can't just learn on levelling-up or similar *, but have to find while adventuring. (In "Level Up" those would be the variant spells; in D&D I'd be tempted to rule that all spells outside of the PHB fall into that category.)I like #9 with the exception that it never gives a spellcaster a chance to try spells they normally wouldn't use. Sometimes, if you find a book with an unusual spell, you might study it just to try it. If I had the choice to turn any spellbook into my own recipe book, I'd just take the spells I had on my wish list. And you might find that many wizards end up taking the same spells...
I'd square that circle by having captured spellbooks do both - they all provide some amount of Lore, and some of them also contain unique spells that a PC can learn. And to encourage the use of those unique spells, I'd probably allow them to be learned at a lower cost. Or just have them be more powerful than 'standard' spells.
* Incidentally, I'd tend to include Clerics, Druids, and Paladins in this as well - they get access to all spells from the PHB "for free", but they need to find the secrets of unique spells though adventuring.