The book where we learn how the Librarian of the Unseen University ended up as an orang-utan. (This happens on the very first pages and anyone who's read at least one Discworld book knows this anyway, so I'm officially not considering it a spoiler.) Other than that, more fun with Rincewind, Twoflower and the luggage, and more send-ups of the1980s' life and times on our round blue planet, complete with Conan Cohen the Barbarian and a doomsday cult. The picture box makes a reappearance, too, and we learn what Death is like when he's at home(show spoiler)
Also, there are dine chewers (say that one aloud). And trolls with a Scottish accent in the audio version. And there's this, on the usefulness of books:
"Cohen was shocked.
'Bonfires of books?'
'Yes. Horrible, isn't it?'
'Right,' said Cohen. He thought it was appalling. Someone who spent his life living rough under the sky knew the value of a good thick book, which ought to outlast at least a season of cooking fires if you were careful how you tore the pages out. Many a life had been saved on a snowy night by a handful of sodden kindling and a really dry book. If you felt like a smoke and couldn't find a pipe, a book was your man every time.
Cohen realized people wrote things in books. It had always seemed to him to be a frivolous waste of paper."
To put this one to optimum use, since it's got the word "light" in the title I'll use it as my book for the Diwali square of 24 Festive Tasks. In addition to which, of course, it is the Discworld group read book for December 2018.