It's hard to describe a Pratchett book to the uninitiated.
Pratchett is English, first of all, with a sense of humor somewhere between Monty Python, Douglas Adams, and P. G. Wodehouse... plus the astringency of one of the fierier Old Testament prophets. Oscar Wilde-ishness with quips. (No other author would admit to his puns.) He's also very very very bright - interested in everything including, increasingly, philosophy.
Disc World's cosmology depends on four elephants and a giant turtle swimming through space. Hard to explain. The (flat) world of Pratchett's stories started as fantasy. As in Tolkien - to whom he is indebted - there are dwarves, men, and other Grimm Bro.s creatures, but it's characteristic of Pratchett that one of his best characters is Death. The anthropomorphic representation. Who speaks in ALL CAPS, rides a pale horse named Binky, and has both a granddaughter and a sidekick who is Death of Rats (also gerbils etc.).
Pratchett's world started as fantasy, whimsy, and humor, but it's not the silly sword-N-sandals stuff people sometimes expect. Even the earliest books are satirical and they've gotten stranger and deeper over the years, blending plot lines, continuing characters, and genres until now Disc World is social commentary and... who knows what?
I suggest starting at Men At Arms, the first book in the sequence featuring Watchman Sam Vimes. Sam may show up in the new book, as he's on the cover.
* CORRECTION: the first Vimes book is Guards! Guards! Start there. (Silly me.)
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