Although it it generally not recommended by fans of the series the Discworld series can be read in order of publication. That list can be found in the drop down box.
3. Equal Rites
4. Mort
5. Sourcery
6. Wyrd Sisters
7. Pyramids
10. Moving Pictures
11. Reaper Man
12. Witches Abroad
13. Small Gods
14. Lords and Ladies
15. Men at Arms
16. Soul Music
19. Feet of Clay
20. Hogfather
21. Jingo
23. Carpe Jugulum
26. Thief of Time
27. The Last Hero
28. The Amazing Maurice And His Educated Rodents
29. Night Watch
30. The Wee Free Men
33. Going Postal
34. Thud!
35. Wintersmith
36. Making Money
39. Snuff
40. Raising Steam
Many fans of the Discworld series prefer to start with a character or a theme and branch out from there. In addition to Pratchett Novels, companion books and short stories are denoted in the graphic created by Kietzman, Oleksow, and Nock.
The Colour of Magic
The Light Fantastic
Sourcery
Faust Eric
Interesting times
The Last Continent
The Last Hero
Unseen Academicals
The Wee Free Men
Hat Full of Sky
Wintersmith
I shall Wear Midnight
The Shepherd's Crown (last book written by Pratchett)
Mort
Reaper Man
Soul Music
Hogfather
Thief of Time
Equal Rites
Wyrd Sisters
Witches Abroad
Lords and Ladies
Maskerade
Carpe Jugulum
Gaurds! Guards!
Men at Arms
Feet of Clay
Jingo
The Fifth Elephant
Night Watch
Thud!
Snuff
Pyramids
Small Gods