I must admit that I haven't read every single book by Terry Prachett. I first heard about him from none the less than Lloyd Alexander who was super-enthusiastic and even gave me his copy of Reaper Man, and I was hooked. Later I read and reviewed the Diggers trilogy, and at one point I had Monstrous Regiment on my fantasy course syllabus. I absolutley love A Hatful of Sky.
I was first a bit disappointed when I realised that Dodger was not a Discworld novel, but I was immediately gripped by the book which was just as clever and ironic and improbable and you-name-it as every Pratchett book. It so happens that I have recently read quite a number of unrelated books that take place in London, and seeing yet another side of it through Pratchett's eyes was exciting. I learned lots of new words, I was tempted to check facts (all quite unncessarily explained in the afterword), I caught myself doing something I interrogate in my research: getting emotionally engaged with the character. I was truly upset when I finished the book. It's an exceptionally high grade for me.