Mods like Apotheosis extend the standard enchantments (and add some of their own) to Level X (10). however the SolarCraft Enchanting Table only allows for enchanting the default levels. I think either a config option in the Enchantment JSON file that specified the max level, or have the mod auto-recognize the max level based on the books that exist would be a great addition that would take not a lot of effort.

For balance, the amounts required to enchant could (optionally) follow an exponential curve.

Thats a good idea, started working on it. (but honestly i have no idea how to detect max enchantment level based on books)

Done.