As a GM just starting a new campaign including a sorceror my solution was to limit the number of spells available in a Grimoire to 5. This helped
to avoid overpowering the mage. Potential discrepancies which could arise through the acquiring of useless Grimoires for some and huge Grimoires stuffed with piles of top class spells for others were also avoided. My game world has sorcerous traditions / traditions which guard their secrets jealously to attract members. As research tends to be into related realms tomes tend to include related spells. For example this is a selection from the grimoires available to a sorceror joining the officially approved Imperial sorcery guild 'The Codex':
Grimoire (The Codex of the Dutiful Ward – Neutralise Magic, Protective Ward, Damage Resistance, Spell Resistance, Spirit Resistance.)
Grimoire (Codex of the Diligent Artesan – Holdfast, Form/Set Metal, Form/Set Stone, Animate Metal, Animate Stone.)
Grimoire (Codex of the Physician of the Heart and Soul – Abjure Pain, Enhance CON, Regenerate, Restoration, Treat Wounds.)
Grimoire (Codex of the Warlock - Attract (Missiles), Attract (Magic), Damage Enhancement, Hinder, Wrack.)
Grimoire (Codex of the Questing Scholar – Fly, Haste, Glow, Mystic Vision, Teleport.)
...and so on. Note that summoning, monster creating and life draining spells are all unavailable and prohibited by this tradition so getting hold of such knowledge would mean looking elsewhere and breaking guild rules!
Our young sorceror hummed and hawed for some time before deciding to divide his skill points between the 'Physician' and 'Questing' grimoires. As part of becoming a ranking mage in the, somewhat distant, future he may create his own named Grimoire containing his favourite spells or discover/create a new spell or spells of his own to add to the guild's exclusive library thus building its power and prestige. I played Ars Magica for many years which has the best magic sytem I've ever seen so, in the unlikely event that time allows it, I might try to come up with rules for discovering and creating new spells as well as scribing and copying tomes.