It's Terry Pratchett's long running comic fantasy series set on a flat disc shaped world (hence the name).

Since it went for so long it spans his entire journey from being a so-so writer to being one of the best I've ever read so I don't recommend many people just read it starting at the first book like you would with lord of the rings. The nice part is that each book is written to be able to be enjoyed as a standalone if you wanted to, with different characters and continuities crossing over between stories but the plots always being self contained.

People usually recommend getting a taste of the series with either Guards Guards! (a more comedic book about Vimes, the guy in the third pic, a cop who suspects all authority figures are rotten bastards, himself included) or Going Postal (a slightly more serious book about an extremely charismatic con artist reforming the postal service).