The following article or section concerns information that is considered even less canonical than the chance of a happy ending. Any information following should not be used as a source for the canon of the book series.
Codebreakers of the Great War is a book by Dag Heath which exists in the continuity of the 2004 film adaptation of A Series of Unfortunate Events.
The book was included in the Baudelaire Library, and read by Klaus Baudelaire. It likely burned in the Baudelaire Fire.
It contained information that later proved useful in deciphering Josephine Anwhistle's coded message.