Heh. Unless I miss my guess, Ashton should start with either Soul Music or Moving Pictures. It'd be out of order chronologically, but my take on his thought processes brings those up right away. He'll especially enjoy Mr. Clete -- dammit, I can't find my copy of Soul Music; I wanted to quote that bit [there's a really neat motorcycle ride in it, too]
After that I'd suggest Mort ["Death comes to us all. When Death came to Mortimer, he offered him a job."] or Equal Rites. The wizard's staff goes to the eighth son of his eighth son, but if you're in a hurry mistakes can be made...
Going back to The Colour of Magic to start at the beginning should be reserved for after one has skipped around in the middle a bit, and the latest ones should not be read without the background.
The Turtle Moves!
(Later edit: Pratchett is much like Gary Larson -- you can tell a lot about a person by which of the books he or she likes best. My personal favorites are Jingo and Johnny and the Bomb)