The Book of Many Things is a new collection for D&D 5e that contains a deck of 66 specialty cards and two new sourcebooks, as follows:

The Deck of Many Things: This an actual set of illustrated cards that are designed to represent the Deck of Many Things itself. It consists of consists of the 22 cards from the original Deck of Many Things artifact (as originally described in the Dungeon Master's Guide) along with 44 new cards.

The Book of Many Things: This 192-page book provides more details on the history of the Deck of Many Things. It also includes advice on using the deck in an existing Dnd 5e campaign, along with adventure hooks, new monsters, character options and magic items inspired by the legendary artifact.

Card Reference Guide: This 80-page guidebook shows Dungeon Masters how to use the cards in an existing DnD 5e campaign. Notably, it offers several different “modes” of use, including using the cards to randomly generate new adventure ideas.