Ok, so: I'm running a game and one of the players is a Wizard, just hit level 6 and she has, through some seriously beautiful roleplay, come to be in possession of the spellbook of a 5000 year old Naga Wizard Lich. The Lich in question is not one of the villains, so I suppose that technically makes him an Arch Lich. The questions is: How is it handled either rules wise or mechanics wise, when one wizard receives another wizards spellbook? Can they cast spells from that book in addition to their own? Obviously of a level that they can cast, but how would that be handled? Side note: I'm using the Variable Spell Casting, Mana rules instead of spell slots.

https://www.google.com/amp/s/olddungeonmaster.com/2017/04/13/dd-5e-mana-based-spellcasting-variant-rule/amp/

Tldr: one wizard found another wizard's spellbook who was a higher level, can they cast spells from it provided they have knowledge of the spell level being cast and or have the mana to cast that spell?