I've read several of Duane's novels, including Spock's World, and really enjoyed them. I'm not wild about the strongly religious bent she gives to Vulcan culture--especially her insistence that (a) there's some kind of real literal god on Vulcan, and (b) Vulcans can perceive it. But I do think the books give a depth and richness to Vulcan culture that's a huge improvement over the sometimes caricaturish image of them in TOS (I don't know about later series).