The Floating Admiral. AGATHA CHRISTIE, DOROTHY L. SAYERS, G. K. CHESTERTON, Charter, 1980.
 
The Detection Club was formed in 1930 by a group of British mystery writers, including Agatha Christie. Written by multiple members of the club, each contributing one or more chapters in turn. In the case of The Floating Admiral, each author also provided a sealed "solution" to the mystery as he or she had written it, including the previous chapters. This was done to prevent a writer from adding impossible complications with no reasonable solution in mind. The various partial solutions were published as part of the final book.
 
Some cover & spine creases otherwise good.