"Etienne" wrote the introduction and explanatory notes in the first edition published by Hutchinson & Co, Paternoster Row, London 1920 (which is my copy - first edition)

It is certainly a novel but possibly/probably written by a former U-boat commander or a senior wach officer.

SMU 122 was commanded by Kplt Alfred Korte from 5.18 - 11.11.18. She was surrendered on 26.11.18 and ran aground on the U.K. East coast in 1921. The wreck was broken up.

The commander also commanded U 55 prior to this command.

I hope that isn't the only book you have read, there are many good ones worth reading.

Hope this is of help

Regards
MPC