Various unique and magical items add 1 or 2 to all spell levels.
Spell levels max out at 15 from reading books.
Casting spells costs mana, an amount that's different for every spell, and which (generally) decreases (to a pre-set hard cap) at higher levels of the spell.
(The amount is equal to the magical requirement to read level 1 of that book.)
Reading a spell book adds a small amount of mana to the available pool.
Spellbooks have magic requirements to read, which increase 20% with each level of the spell.