The Discworld books are only a "series" by the second-weakest definition (books set in the same world by the same author; the weakest is merely books set in the same world -- considering all books set in the Forgotten Realms or in the Star Trek universe to be part of the same series seems a bit dicey). I mean, exactly what does Mort have to do with Lords and Ladies or The Truth? Pratchett has four series set on the Disc (Death, Rincewind, Watch, Witches, plus the standalones and the YA books), but they're not (for the most part) tightly linked books.