In the global chat, Swen said it's something like a level 14 skill now, so it's not sold in Cyseal. That's probably a good thing, because when it was sold for the cheap price of "Witchcraft 2", basically everyone picked it up.
Missing Skillbooks? You won't be able to just go out and buy every skillbook for every skill right from the start. Skills and skillbooks have levels themselves, and will start popping up when you reach that level, or when you are near it. If anyone in Cyseal would sell a level 14 skillbook, it would be very expensive, you may not be able to meet the requirements, and the AP cost to cast it might be too high to be realistic (since your Intelligence rating may still be too low).
All skillbooks up to level 9 are available in Cyseal. The other 49 skills are available after Cyseal or indeed when you craft them yourself.
The first elemental summon skills, are the Summon Fire Elemental and Summon Ice Elemental, and those are level 6 and level 8 skills, so they should be available in Cyseal. (Arhu sells fire skills. The magic marketstand sells Water skills.)
In short: skills are spread throughout the game so that you feel your caster becoming more and more powerful.
