Marking this as "read" is a bit of a mis-shelving, since one does not simply read, cover to cover, a book like this. It is an encyclopedia of the Discworld and its residents, up through the events of Snuff. Unfortunately, it straddles an uncomfortable middle between being an in-world encyclopedia and being an authoritative source, and it ends up with some rough edges.
Some of the entries have detailed notes about which particular books a given person/event/etc appears in, in order. Others do not. Some entries go into lots of specific details, others read like a blurb on the back of the book [complete with jokes and teasers about what is inside]. There seems to be a bit of a lean towards the more recent books, with [say] Adora Bell Dearheart having a more specific entry than Susan Sto Helit despite the latter having (up through Snuff, anyhow) a more relatively prominent place in the Discworld canon. And some of the entries would be a bit of a bear to find, such as those dealing with the animals of the Disc.
Still, it's a fair read and not a bad way to refresh the memory about the various entities and namesakes of the Disc, you'll just have to work to get a complete picture.