Unless you have a frighteningly gigantic INT or a pathetically tiny repertoire of spells, you will need some sort of spellbook to store all the magic you don't have in mind right at this very minute.
Even if you don't have a repertoire large enough to overflow your cranium, a spellbook is a good backup in case of a blow to the head, since a nasty knock (an Impairing wound to HitLocs 3, 4 or 5) can cause you to forget any or all of your spells. This might not be permanent, but then again...
Even calligraphically-challenged magicians can make use of a generic Spellbook. A tribal shaman, for example, might have a bizarre cats-cradle of string, feathers and shells which acts as a mnemonic aid to remembering how to make the chief supremely potent and his wives equally fertile, so that the numbers of his children runs into the hundreds. Just because it's called a Spellbook™ doesn't mean that it has to be a book, per se.
A spellbook can be a focus for a Power Pool, or (if you have to cast the spells directly from the book), for the spells themselves, or both. The nature of the Focus will depend on how you define the generic Book's special effects.
For example, a list of spells carved into the polished granite walls of a wizard's tower, from which he learns (but does not cast) his spells would be an Immobile Inobvious Inaccessible Focus, for a total limitation of -1¼. Inobvious because the powers do not obviously emanate from it when he casts them, but it is necessary to have access to it to be able to use the powers. Immobile and Inaccessible because it's a wall, for crying out loud - you can't take it away from him in combat and he can't move it around.
At the other end of the scale is the wizard who - god knows why - has written all of his spells in plain language that anyone with a magic skill can understand in water-soluble ink on dried oak-leaves carried in the same balsawood box as a half-gallon jar of naphtha (because it keeps the insects away). He has to cast the spells directly from the leaves because he has a terrible memory and couldn't possibly remember them, and they glow with a Mystic Aura while the power is in effect. This would be a very short-lived Independent Fragile Obvious Accessible Focus - I think you can work out why. It's worth a heap in terms of limitations (-3¼), but is it really worth saving all those points when you're only going to have any spells for the first five minutes of the game? I don't think so.