Enchiridion

A small book of instruction, used by practitioners of Internal Magic to learn spells, or to learn higher level spells. These books can only be made by External Magic users, though the process varies by the individual. Because of the rarity of External Magic users however, these books are not easily obtained.

Each book can only be used once before it disappears, as well as erasing any knowledge of any book before it from the readers mind.

Many, if not all, spells have multiple levels called "chapters". There is a separate book for each chapter, and nothing happens if you try to read a high level book without already knowing the previous level.

Each book cover shows the name of the spell, it's chapter number, and how much mana a single cast would consume. If there is also a maintenance cost that is shown here as well (cost to maintain spell per second).

Some spells might have a casting cost of 10 mana and then cost 1 mana per second or something. I dunno yet.