I only half agree with Alex on this.
It is fairly clear (to me anyways :? ) that a Sorcery (Grimoire) skill represents a particular philosophy, a way of seeing the underlying currents of the world and understanding how to manipulate these with sorcery.
The scenario Alex describes presupposes that any new spell created by a master sorceror would by necessity have to involve the creation of a new Grimoire.
I propose that it is far more likely a sorceror will create or develop a new spell or effect within a sorcerous philosophy he is already familiar with, expanding his current Grimoire and relying on his current skill, rather than inventing a new philosophy from scratch.
I could see this being used to both add existing Core Book spells to a Grimoire that does not have them as well as truly creating a new spell effect.
There admittedly would have to be significant GM involvement regarding the appropriatness of the spell being researched/added. I could see a GM ruling that a master diabolist, expert of the Sorcery (Book of a Thousand and One Blasphemous Names) skill, could not realistically create the spell "Peace, Love and Harmony" and add his to his Grimoire.
Therefore the mark of a true Master Sorceror, as Alex says, would not be to create a new spell effect, but rather to create (unveil really) a new philosophical understanding of the world that would allow for an alternative Sorcery (Grimoire) skill, perhaps broadening the spectrum of spell effects available to him.
Please note that what I mean by Grimoire in the paragraphs above refers to the philosophical underpinnings of the sorcery, rather than the physical book or tablets or whatever. Whether the sorcerer has "blank pages" to scribe his spell on or has to buy a new book has no real bearing on my point. A single "Grimoire" can be an entire library as far as I am concerned.
Alternatively, go with Simalacrum and put everything under one single Sorcery skill
Have fun,
Dan