So, let's see... So far we've read Monstrous Regiment, Guards! Guards!, Going Postal, Small Gods, Night Watch, Jingo, Men at Arms, The Fifth Elephant, Thud!, Feet of Clay, Reaper Man, The Truth, and are working on Soul Music.
I must say that, though they're all fun reads, he really hits his stride stylistically about the time of Night Watch or Thud!.
Oh, and to those of you who let me go sit on the porch with a beer to settle in and read the last seventy-odd pages of Reaper Man next to the fire without warning me to bring a box of Kleenex, you suck. Running across something that deep and poignant in the middle of an otherwise normal Terry Pratchett read is like finding the ending of Old Yeller in the middle of a screening of Monty Python and the Holy Grail.