When Shinwell opens the trunk at the beginning of the episode, the hat he pulls out is a deerstalker hat, an homage to Sir Arthur Conan Doyle's original version of Sherlock Holmes.
Sherlock says that five years ago he borrowed a book from the library and never returned it: the Codex Seraphinianus by Luigi Serafini.
The Codex is an illustrated encyclopedia of a imaginary alien planet, written in an imaginary language.
Despite being unreadable, it is a much sought after and expensive book, with even the most recent prints going for hundreds of pounds.
The author has stated that the book is a game, written purposely to be indecipherable. When asked about a possible translation, Mr Serafini said his work should only ever be translated into another imaginary language.