It seems to me that someone wrote in a book that Welles couldn't have seen "If I Die..." in a shop window in 1946 when he was talking to Harry Cohen on the phone, and thus immediatley suggested it as a basis for a movie, because no paperback had been released yet...
Well here's an ad for the paperback: a 1940 pressing:
If I Die Before I Wake
Bookseller: Nightingale Books
(Stoughton, MA, U.S.A.) Price: US$ 275.00
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Book Description: Mystery Novel Of The Month NN (#16), New York, 1940. 1st Paperback Ed. Near Fine in red pictorial wraps with black & orange lettering & cover photo of a convict in stripes looking out of prison bars. Mystery novel about an Irish adventurer who joins a seductive woman & her husband on a Pacific cruise. Basis for the 1948 classic film 'The Lady From Shanghai' starring Orson Welles & Rita