Hm. They are YA, so they're not quite as challenging or dense as Pratchett's other Discworld books, and Tiffany is a standout character (so determined and flawed!). They mention other Discworld characters, but you don't really need to know them to enjoy the Tiffany books, as long as you are ready for Pratchett's parodist, over-the-top style. I'd say give the first one a try and see what you think.