By Kistandalius at the Rivertown Market you get the most luck he's selling a skillbook .
But there are more NPC's to check, even a NPC that do not look like a merchant .. in fact, with most NPC's you can open the trade window .
But I specially look for equipment items (rings, amulets, leggings, gloves, etc) - rather than searching for skillbooks - that have a bonus to a skill, sometimes 2 different skills on one piece get a bonus
.
Good luck
