So, spells originally come from demons. Many demons have the ability to project their consciousness outwards from their bodies, manifesting as fireballs or choking mists or whatever. Unmoving, vacant gazed, their bodies wait until the consciousness returns (usually quite quickly). Some people a very long time ago figured out how to trap those roving consciousnesses, first as a defense, then utilizing them as weapons. All spells are therefore unique entities, although there is some commonality in function between them.
|I don't know what level spell this guy is but it's bad news [src]|
The spellbook mind-traps will collect the dispersed consciousness of a spell after casting. Scrolls are easier to make since they don't need the functionality of the re-trapping, the consciousness is just set free permanently.
You can gain new spells by:
Stealing from someone else
Making a contract with a demon for the temporary use of its consciousness
Overpowering a minor demon and taking its mind
Killing other spellcasters and taking the spell etched on the inside of their skull
Breeding new ones (the most common method)
Spells on the insides of skulls: From Logan Knight here. Sorcerers, in order to cast spells, warp their consciousness into a prison, becoming more spell- and demon-like themselves in the process. At the end of their lifespan their mind partially congeals into a unique spell etched on the inside of their skull - the effect being proportional to their power and related to their subconscious desires or obsessions. Pyromaniacs usually result in fire spells, paranoid schemers an insidious fear spell, lepidopterists a hurricane of butterflies, etc. The spell-mind retains a fragment of the personality and secrets known by the person. Basically, cast your dead friend/enemy once a day.
Breeding new spells: First you gotta transfer the spell-mind to a corporeal body. Then get it to reproduce. Made bodies are used for this process, special empty-minded homunculi created to mature and reproduce rapidly [edit - vatlings]. Arms, eyes, stability are optional since it's just a temporary vessel, though the size of the body does need to be proportional to the spell's power. 1/6 chance of the spell mutating at this point, per Arnold's table. 1/12 chance of the spell mutating such that it loses the ability to cast and becomes inert. When different spells interbreed there's like a 3/6 chance they don't mix right, and something weird and bad happens. Like terrible mutants, spell fallout, or what-have-you. Homunculi require expensive ingredients, and more problematically, an initial sample of demon-flesh. It's a lot easier to get this initial sample from other sorcerers than the source, though that dilutes the potency.
|A temporary homunculus [Berserk]|
|Level 1 spell homunculi? [src]|
Escaped spells can be problems. Gobbos are presumed to be escaped spells that have lost their ability to cast but retained their artificial fecundity intended for lab use. Higher-level spells that are ogre-sized are a more deadly threat. Even more deadly when through mutation a strain re-gains their ability to cast themselves. Feverlings happen like this too.
Spells take on personalities and gain stronger wills the longer they've been thought. A 1st level spell doesn't own a large share of your consciousness and so hardly matures at all. A spell with its own body matures much more rapidly. One can only imagine the plight of mighty sorcerers trying to shut out the voices of trapped elder demons. The bargains to convince the spell to be cast must be dire indeed.
Gaining new spell-effects: Other than mutation or getting spells from the wild, some sorcerers master a drug-induced hallucinatory trance that allows them, at great physical and emotional strain, to alter spell-minds currently memorized. This causes loss of XP as areas of the mind are broken down in the process. It's a strong mental poison that alters all within the mind. For the comparatively smaller spell-mind, this means a proportionally more total change, but there is a cost to the host as well. The trance can be controlled somewhat through extensive preparations.
Demons hate sorcerers especially because sorcerers are all running around with trapped fragments of demon-minds in their heads. Killing sorcerers has that extra relish of vengeance. Killing sorcerers probably releases/casts all their spells at once, so watch out for that I guess.
Also I just got WFRP 1e and it's awesome.