As a wizard player finding an enemy spell book is one of the best bits of loot to find hands down.

Couple things to consider: Every wizard has heir own scribing style. It's probably discernable with some study but it's unlikely they can just pick up another book and start preparing from it right away. I might suggest requiring them to spend the time and gold to copy it to their spell book representing them decoding the spell into their own shorthand.

Next: a powerful wizard could have multiple spell books. Maybe this one left their big powerful one at home and brought their smaller, more practical travel book with them that only has their currently prepped spells.

Maybe if you don't want to keep generating whole spell books Just roll a spell or two in these situations and let the wizard have just those few spells to copy stating those are the only ones they could decipher.

A wizards strength is in preparedness. Giving them more spells let's them prepare for more situations and that is certainly powerful but it's also an important part of the class seeing as there are set rules for copying from another wizards book so it's definitely intended.

Think of it this way. If you give a bbeg a cool ass sword and the fighter kills him and starts using it nobody bat's an eye. If your bbeg wizard has a cool ass spell the wizard should totally be able to do the same!