There is an immense amount of detail about the German Submarine Warfare from 1914-1918 in this book: about the technical design of the German submarines, their officers and men, their tactics, their cruises, their victims and their opponents.
This book is completely based on British sources from the National Archives, Kew, UK: Prisoner statements, Intelligence analysis, deciphered W/T, reports of sunken ships.
"[U-Boat Captain Weddingen] invited me to come aboard the Submarine. He asked me to have a glass of wine and which I accepted. The men on deck were all well, fit, happy and appeared to be enjoying themselves the same as if they were on a picnic."
Capt. J. Williams, Master of S.S. INDIAN CITY, torpedoed 12th March 1915.
"On board the ship arises great confusion; some boats full with people, rush downward, struck the water with the bow or stern first and are filled with water at once."
U-20, Kptlt. Schwieger, 7 May 1915, Sinking of the LUSITANIA.
"As the water continued to rise in the torpedo room, the men gave up every hope of escape, and to prevent slow death by suffocation some committed suicide by putting wadding in the nose and mouth and throwing themselves into the water in the boat. Others tried to shoot themselves, but the pistols had become wet and did not fire."
Survivors of German UB-55, sunk 24 April 1918
"Oblt. Dönitz. At first refused to answer any questions whatever, and even had to be persuaded to write his name. As he had previously said he was done with sea and ships, it seems probable that the loss of UB-68 was due to a direct fault of the commander."
Interrogation of Survivors of UB-68, 4th Oct. 1918