They do get new spell books, depending on your level and the merchant in question. There are more merchants in the next town that you get to, as well as in the homestead, once you open the appropriate portals. The next town should be available by level 10 (you could get to Luculla Forest now if you wish, going through the fire area north east of town; one opponent might be an issue depending on party composition and difficulty, but you can sneak to get someone past it). Depending how thoroughly you explored the Black Cove, you may have access to the first elemental portal in the homestead by then, as well.
