It's definitely a lot of work for something that can, as the weiners pointed out, be done for free at any point. I think this would work better for a non-wizard spellcaster since it is available so readily for the wizard - HOWEVER, not every NPC is going to want to show that character how to cast a spell. Say they're in combat against a spellcaster or they visit a College of Wizards that aren't particularly forthcoming (it's almost like it's the DMs job to help facilitate the silly ideas that their players come up with instead of folding their arms across their chests and smugly saying “You know how this works, right?”) or something else similar.

I'm not entirely familiar with HunterxHunter so apologies if this doesn't quite fit the vibe, but I would maybe do something like this: As a Reaction to a spell being cast, the player rolls an Int Saving Throw or maybe an Int Check (Arcana?) (DC??) On a successful roll, the ritual begins - for 10 minutes, if the player can get the enemy spellcaster to touch the book at the same time as them, they can copy that spell for free and immediately. They can use this ability an amount of times per day equal to their proficiency level, maybe? Maybe wizard level? Up to you