There are different 'groups' within the Discworld series. The 'groups' are written inside the parenthesis. Some books are part of more than one group. The titles highligted in red are indicate the starter book for group.
3. Equal Rites (The Witches)
5. Sourcery (Rincewind)
6. Wyrd Sisters (The Witches)
7. Pyramids (Gods)
8. Guards! Guards! (The City Watch)
9. Eric (Rincewind)
10. Moving Pictures (The Wizards)
12. Witches Abroad (The Witches)
13. Lords and Ladies (The Witches, The Wizards)
14. Small Gods (The History Monks, Gods)
15. Men at Arms (The City Watch)
16. Soul Music (Death, Susan, The Wizards)
17. Interesting Times (Rincewind, The Silver Horde)
18. Maskerade (The Witches)
21. Feet of Clay (The City Watch)
22. The Hogfather (Death, Susan, The Wizards, Gods)
23. Jingo (The City Watch)
25. The Last Continent (Rincewind, The Wizards)
26. Carpe Jugulum (The Witches, Uberwald)
27. The Fifth Elephant (The City Watch)
28. The Truth (William de Worde, The City Watch)
29. The Thief of Time (Death, Susan, The History Monks)
30. The Last Hero (Rincewind, The Silver Horde)
32. Night Watch (The City Watch)
33. Monstrous Regiment (William de Worde, The City Watch)
34. Going Postal (Moist von Lipwig)
35. Thud! (The City Watch)
36. Making Money (Moist von Lipwig)
Click HERE to see Wikipedia's list of books, published date and motifs.
For Discworld Reading Order Guides (really cool concept maps) go to L-Space. Be sure to check out the Graphic and the Excel. Very nice!
Terry Pratchett's website.