I very much recommend the unofficial biography "The Magic of Terry Pratchett" by Marc Burrows as an accompaniment, as well, which was written from a different perspective. Although 'unauthorised' it did have the approval of both Narrativia and the Pratchett estate with both Rob and Marc doing a double act on stage at the Convention.
(I admit that I am slightly biased since one of my photos appears in Marc Burrow's book.)