"Buyer Beware..."
Once you get hooked on Terry Pratchett, you can kiss your membership credits goodbye. I started with the Tiffany Aching trilogy: A Hat Full of Sky, The Wee Free Men, and Wintersmith. I loved them, especially the first two. Next, I listened to Nation, another excellent book. I think that all of these books are actually set in the Discworld, though it isn't mentioned in the books.
I went searching for more Pratchett novels after burning through the Tiffany Aching series, and wouldn't you know it, there is a 30+ book series called Discworld. Pratchett's stories are so fun to listen to and very engaging. His imagination and sense of humor (or "humour", since he's British) are addictive! I am currently on Discworld book #3. I have already downloaded 4 others. I now restrict myself to listening to Discworld books only when I'm away from home. Otherwise, I would be through the entire series within a few months.
I recommend starting the series in order, until you understand what the Discorld is all about.
As others have mentioned, the narrators are incredible! Stephen Briggs and Nigel Planer do a bang-up job narrating. Even Celia Imrie is enjoyable, once you get over the fact that she is neither Planer nor Briggs. Try to appreciate her for who she is rather than who she isn't (If you are new to Pratchett, those narrators will become familiar to you soon enough, even if you don't consider yourself a narration snob.) Then all of the reviews that mention them will make sense.
I've been ill, so forgive me if I'm babbling nonsense. I renewed my Platinum membership in August and am already down to 17 credits. Thought it only fair to warn you. Enjoy!