I read The Bromeliad (Truckers, Diggers and Wings) for the first time and now I can totally see why it's so many people's way in to Pratchett. Incidentally, if you'd like to see my recommendations for where to start if you've never read any, I wrote a post about that!
The basic premise of the trilogy is that our world are full of little 4 inch high creatures called nomes who move so fast that humans have no idea they even exist. The story begins when the few nomes left living outside, led by Masklin, leave their home on the back of a truck and arrive at 'the Store.' The store nomes think that the store is the entire world and that its founder, Arnold Bros (Est. 1905) is God, and the nomes from outside have their work cut out for them to fit into their new surroundings!
All three books are very short and packed full of Pratchett's trademark satirical wit. Personally I loved how well big ideas were slipped in while making them seem like just part of a book for kids. I also loved the subtlety of the story, and towards the end of Wings particularly there was a really lovely bit which made me go 'aaaawwwwww'. If you enjoy Pratchett's writing you should definitely read these books. If you've never read any Pratchett, Truckers is a great place to start.