[LFFF]

it could be the reasoning for memorised spells actually. Your big spellbook could be the livelihood, the bases of all you learned, but in the process of memorising spells, you could actually just be copying what spells you wish to bring with you onto a smaller spellbook, when you cast the spell reading from the smaller book, the page could dissapear or something, and when its all done, you will once again have to go into your big spellbook, whether you carry it with you or leave it somewhere you deem safe and copy what you wish to memorise into another smaller book.