The never-filmed Alraune und der Golem announced for production in Germany in 1919 has been discussed around here before, but nestled in threads about other movies (such as here and here with some good background information on the film), so I'm starting a new one.
Alraune und der Golem was planned as a filmatization of Achim von Arnim's novel "Isabella von Ägypten" ('Isabella from Egypt'). But another fact, and I don't know how well known this is, is that a novelization of the meant-to-be film was published in Germany in 1921 with the same title as the film. The author of this rare book is Richard Kühle, who supposedly was the script writer for the film as well. The novel is introduced by the text "Ein phantastischer Roman nach dem gleichnamigen Kolossal-Film der Rheinlicht-Bioskop bearbeitet von Richard Kühle" ('A fantastic novel after the colossal film of the same name by Rheinlicht-Bioskop adapted by Richard Kühle').
Independent publisher Goblin Press has reprinted the book (in German only - link) with a 10 page well-researched epilogue on the novel, author, film and people behind it.