Sherlock: The Casebook, by Guy Adams, is a piece of Sherlock merchandise in the form of a book which covers and provides more detail on the cases (episodes) of the show.
It is written in an in-world style, as if it had been written by Dr John Watson with additions from Sherlock Holmes. The book also includes interviews with Benedict Cumberbatch, Martin Freeman, Andrew Scott, Steven Moffat, and Mark Gatiss.