A) Where to Start
Although each novel has a number, they’re actually grouped into different series that at times reference eachother, but more or less exist as their own tales occuring in the same world. There are handy graphics out there that show which books belong to what series and in what order, like this one here. They all have that same great T. Pratchett wit, but the characters and themes are different, so where to begin is really a matter of what your preferences are.
City Watch follows the life of Sam Vimes and the city guards of Ankh Morpork, as they go from a small, largely inept band of maligned wash-ups to a well-functioning police force in an increasingly modern city. It’s everything I love about crime drama and high fantasy meshed into one. A conspiracy involving a werewolf family and dwarven royalty, a series of murders that point to a dragon as the culprit, finding a place for gnomes and vampires in the police force… all great stuff. Also, some of Pratchett’s most poignant satire.
The Witches novels follows, well, the witches of the Discworld, primarily the begrudgingly-good Granny Weatherwax, jovial matriarch Nanny Ogg, and “wet hen” Magrat Garlick. These books tend to be lighter in tone (so far) and poke fun at fairy tales and the various portrayals of witches in English literature. Anyone can enjoy them, but they’re much more amusing if one has a working familiarity with Shakespeare as well as fairy tales. My personal favorite of these is Wyrd Sisters, which draws a lot from MacBeth.
The Death Novels are about Death the character, who appears in almost every Discworld novel at least briefly. They’re great, but it’s best to read at least a few from any of the other series so that you’ve seen the character before jumping into the series about him and his family/assistants.
The other series I haven’t touched yet, so I can’t say much on what’s good about them or what sort of person would like them most, but I’ve no doubt they’re great, and you may want to read more yourself on what they contained. Rincewind or Tiffany Aching might be more to your liking, you never know!
B) Where to Find Them
There are e-book and audio-book versions of the books on amazon if you’re not interested in a physical copy. Personally, I really like the audiobooks, because I can enjoy them anytime anywhere and am free to doodle characters while I do, and the readers are all very good.
If you want to find them for free, well… I can’t say anything about that other than that there is definitely is a place somewhere where you can probably find some of them.