Mayfair Games had a 3rd party supplement for 2e called Archmages and they had powerful artifact level grimoires, what you are suggesting is nothing compared to those.what I would do is 'borrow' some ideas though. Have the book take time to 'read' and 'learn' something like "You need to take a week to study it and make a DC XX Arcana check to understand it. When you do you get a +1 to the DC of all your spells. After that you can spend 2 more weeks studying it and make a DCXX+2 check to gain a +1 to all attack rolls with your spells. After that you can spend 4 more weeks studying and make a DCXX+4 check to gain +1 to damage with all cantrips. After that you can spend 8 more weeks studying and make a DCXX+6 check to up that +1 to DC to +2. After that you can spend 16 more weeks studying and make a DCXX+8 check to up the attack bonus to +2."
that is 31 weeks of study of an artifact that goes with "you need to be smart and study the book" as the fluff but it also gives you the ability for that item to 'grow' over time with you (unless they take more then half a year off all at once).
I can even say adding a spell not on the wizard spell list to the wizard spell book, letting a 3rd level or lower spell become a 1/day and/or always preped for you. letting a 1st level non damaging (non shield) spell become a cantrip for you. grant you a feat like elemental adept. upgrade a damage die of a spell (like your magic missiles are now 1d6+1 instead of 1d4+1) are all OTHER example things such a book could grant.
and no, level 7 is a perfect time to allow a artifact, but you should have some RP down sides...
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