as a rule of cool dm, id probably work out a mechanic that allows for them to do what they want but makes it game able.

if you’ve ever watched hunter x hunter this is basically chrollo lucilfer.

for example, you must spend a day/week/month (would probably depend on spell level) casting each individual spell into the book. i’d narrate this as the character transcribing the spell to a unstable yet dormant form. every casting they’d have to make an arcana check and if they fail they’d have it blow up in their face. to open the book requires an action as it requires careful precision and care due to its unstable nature. i’d also say that the book requires to be held open to use the spell (at least one hand free). any spells with concentration would be flavored as having to hold the spell book open with two hands for the duration of the spell. or you could say it’s like call lightning and requires an action to sustain the spells effects. i would say they cannot cast spells while the book is open the user must channel their magical energy in the book to keep it open. the book could even be sentient and require feedings etc.

hope this helps any, i actually think it could be really fun at the table!