From Discworld & Terry Pratchett Wiki
The Discworld Series consists of several "sub series". These centre around certain (groups of) characters, for example, the Watch Series centres around the Ankh-Morpork police force. The books aren't labeled according to the series they belong to, and the books can be read and enjoyed without reading them in order (phew!). But to get the most pleasure out of them, reading the books in their proper order is best.
Recommended starter books
Which book to start with is a common question, with many answers. There is a difference between earlier books as opposed to later books and the balance swings from "lighter" to "darker" and back through the series.
Generally, the following books are recommended to start with:
- The Colour of Magic is the first book, but some people consider it less suitable as a starter because the writing style differs greatly from later books. Some people might prefer it, others will probably enjoy later books better. The book starts the popular Rincewind Series.
- Mort, the 4th Discworld Book, starts the Death Series
- Small Gods, a stand-alone novel.
- Guards! Guards! starts the Watch Series, a natural for fans of detective novels, police procedurals and film noir.
- Equal Rites starts the Witches Series, although it doesn't include most of the cast beyond Granny Weatherwax, so Wyrd Sisters might well be used as a starter. Equal Rites can also introduce Ankh-Morpork.
- Most recently there's the Ankh-Morpork Series, describing the modernisation of the city under Patrician Havelock Vetinari and charting the adventures of the former con-man Moist von Lipwig and newspaper founder William de Worde. This starts with The Truth.
- The Wee Free Men is the first book of the Tiffany Series for younger readers (not dead yet, at least).
Official Reading Order Guide
In 2015 HarperCollins released an official reading guide for the Discworld novels, though rather than suggest a full order to read the books in, it links the works that are most connected e.g. the Witches series.
Some expert advice
"...I find it now rather embarrassing that people beginning the Discworld Series start with The Colour of Magic and The Light Fantastic, which I don't think are some of the best books to start with. This is the author saying this, folks. Do not start at the beginning with Discworld." (Terry Pratchett in 'Straight from the Heart, Via the Groin', included in A Slip of the Keyboard.)