The book was intended to help whoever found it learn to fill the shoes of the Doctor, protector of the universe, until the real Doctor could find her way back. It also included an introduction by the Doctor and brief profiles of herself and her companions. The Doctor's symbol marked out specific passages wherein which the Doctor had appended "hints and tips" to the challenges laid out by the book. (PROSE: Meet the Fam)
The metafictional implication is that the book written by the Thirteenth Doctor is none other than Doctor Who The Official Annual 2021 itself, with the challenges preparing the reader to become an "interim Doctor" being the games and activities contained in the book on top of various prose stories.