In order to provide access to a variety of spells via scrolls, but not create easy means of utilizing that, scrolls would not be capable of teaching spells to the user.  The scroll is in effect a cliffs notes and while sufficient to cast the spell to its ultimate completion, it is however not enough to come to truly know and understand the spell.

In that way to learn a spellbound require a complete copy of the spell.

This will be what is normally found in spell books.  Spell books contain the whole pattern for the spell.  However it likely does not hold sthe stored power as a scroll does.

   Suppose you give an ancient scroll with a "lost" spell you could research the scroll to find out how to write the spell in your spell book.  But that wouldn't be the same as "copying" the spell.  More expensive, more time, and maybe some plot points finding/testing material components and inks that are no longer common.

Considering the following:

Increasing the number of Skill points given to Wizards, but in doing so, they are subject to a new requirement, where to 'learn' specific spells one must meet some additional prerequisites, normally in the form of one or more skills.

Alternately, caster levels might get tied to certain skill ranks, but that seems more extreme and potentially unreasonable, except for potentially rare, generally unknown specialized spells.


[Default requirements by type]
Unless specified any skill required, requires 1 rank per spell level.
Abjuration:  Knowledge: Arcana
Conjuration:  Knowledge: Planes
Divination:  Knowledge: Arcana or Perception
Enchantment:
Evocation:  Knowledge: Planes
Illusion:  Craft: (Painting, Sculpture, Drawing, etc.)
Necromancy:  Heal (1 rank per spell level) or Craft (Body/Flesh) or Profession: Surgeon
Transmutation:  Craft: Any or Knowledge: Engineering


[Specifics/Examples]
Summon Monster I, Knowledge: Planes 1 rank
Summon Minor Monsters, Knowledge: Planes 1 rank
Obscuring Mist, Knowledge: Nature 1 rank

   I have some competing ideas, one a Feat that allows one to develop special material components, even for spells that don't use them.  that allow a "magic user" a bonus based on knowledge and craft skills.

   The price is that the components are rarer, more expensive, and require the User to spend "plot points" i.e. must be 'Taken from a Graveyard during a full moon' etc.

 Another is a prestige class that does allow learning more skills, while linking skills to spells.  Though I don't know if the P. classes can work that way.  This makes me wonder suppose if you have the Illusion spell you may spend a 'special prestige skill point' to gain a appropriate skill(Painting, Sculpture, Drawing, etc.). but if you do they become linked to that spell and you must roll a skill check with the spell.

Prestige classes can function as any class.  They basically are like the old fashioned Bard class, and what I think I used to call Compound Classes.  Basically being a class that has 'Prerequisites' for entering into it.

So what you would need to design, would be a Class Feature, which would be unique to the Prestige class.  Actually, it would seem that what you are actually inventing is a form of metamagic feat (or set of them) that would have requirements tying them to the prestige class.

I'm trying to consider what you could use the skill to help with.  I guess, actually, there are a variety of options on what you might be able to do.  A couple that come to mind right away include, raising the effective caster level of the indicated spell.  Another option would be increasing the DC, of the save, of the indicated spell higher than normal.  Another thing I know is sometimes desired is that ability to have a bonus to penetrate spell resistance effect.

I can see each of those being a potential option for something like that.

Perhaps, for instance, making an appropriate craft roll, and paying some cost similar to a scroll or such, would allow you to make a material component with a particular effectiveness recorded.  If using that prepared material component to cast the spell, there would be some process to compare the crafted object against the target, and if greater, affects the spell in some designated way.

Things I can think of... Result of Craft Roll must exceed:  WILL, REFLEX, FORT save DCs, or CMB( Combat Maneuver Defence) for some examples.  Not sure I have perfect ideas at the moment for them all though.  But I could easily see using such a material component, being able to say improve the DC of the spell by a +2 (which would be significant) thinking of it as sort of being similar to a 'assist other' on the spell casting.  A similar bonus of some sort to affect Spell Resistance Penetration also seems reasonable.  These material components however would probably have to be prepared for a specific function (boost DC, or boost penetration)  Actually, crafting special components could each be a different meta-magic feat.  The prestige class would pick them up in a specific order, or you could allow them to pick one up per level or x/levels.