The skill books that the vendors sell are randomly generated within the level restrictions of the area that you're in (i.e. Cyseal skill books shouldn't go above level 10).
Due to the random inventory generation for merchants, you may have the bad luck of not finding what you're actually looking for. Just go out, murder some dead and come back for them to refresh their inventory. Note that I don't know exactly what triggers merchant inventories to refresh. It could be time-based or it could be enough to just go far enough away and then return.
