Actually you could have 6 books, for each school, then maybe divide each as lesser, normal and greater. So let's say making a normal tomb of evocation, you would need:
2x scrolls of fireball
2x scrolls of magic missle
2x melf's acid breath
2x scroll X
Gemdusts
Leather
Gems
And it would cast each of the spells let us say 5 times. Also require spell focus to craft the lesser and normal tomes and greater spell focus to craft the greater tomes and then limit their use to wizards and bards only.
The above will accomplish, that tomes will be meant for educated spell caster as a means of increasing the number of spells they have. They will be hard to make and will be limited to the wizards who take the school seriously. They will also make an interesting RP goal for rogues as I can hardly see a mage, who studied his entire life, who would put his knowledge in a book and hand it to some Joe who happened to know arcane runes.
As a final point, I see this as a good idea, would certainly lighten scribing up and make it more interesting. And it should go into scribing, as not everything with charges automatically equals infusing. They will not be overpowered, as items usually have low DCs and casting levels and even the greater books will be limited by level 5 spells.