Posted this on another thread on the same topic but think it might help. Daddystabz's is right enough although I feel a bit of flavour and style could be added. My solution was to limit the number of spells available in a Grimoire to 5. This helped to avoid overpowering the mage (In fact my Grimoires also function as research and training treatises. So if read for a week they can grant a number of improvement rolls in related lores).
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 preference was that players could not just pick the most powerful spells and stick them in their book. My setting has sorcerous traditions which guard their secrets jealously to attract members. As sorcerous research tends to be into particular natural or mystic realms tomes tend to include related spells. e.g. 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.)
Improvement Rolls: Lore(Sorcery), Perception, Manipulation, Insight, Persistence.
Grimoire (Codex of the Diligent Artesan – Holdfast, Form/Set Metal, Form/Set Stone, Animate Metal, Animate Stone.)
Improvement Rolls: Craft Stone Craft Metal, Mechanisms, Engineering, Alchemy.
Grimoire (Codex of the Physician of the Heart and Soul – Abjure Pain, Enhance CON, Regenerate, Restoration, Treat Wounds.)
Improvement Rolls: First Aid, Resilience, Healing, Meditation, Survival.
Grimoire (Codex of the Warlock - Attract (Missiles), Attract (Magic), Damage Enhancement, Hinder, Wrack.)
Improvement Rolls: Close Combat Style, Ranged Combat Style, Manipulation, Lore (Battle), Evade.
Grimoire (Codex of the Questing Scholar – Fly, Haste, Glow, Mystic Vision, Teleport.)
Improvement Rolls: Ride, Swim, Drive, Boating, Shiphandling.
...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 forbidden knowledge would mean looking elsewhere and breaking guild rules! Players may have to go out looking for particular spells.
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 distant future he may create his own named Grimoire containing his favourite spells or discover/create a new spell or spells to add to the guild's exclusive library thus enhancing 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.