Submarine is my first purely naval book. My aim was to present a complete picture of what it was like to be a submariner, under all circumstances, during the Second World War, irrespective of  nationality, rank and wartime allegiance. The submariners tell their stories in their own words, describing all the hardships of life on board and the dangers that faced them from mines, depth charges, torpedoes, mechanical failures, friendly fire and bad weather; they talk about the operations in which they participated – sinking enemy shipping, landing agents, laying mines, sneaking into fortified bases . They also describe the terrors of escaping a doomed boat, and encounters with the enemy, whether as prisoners or rescuers. The accounts range from the very funny to the dramatic and deeply moving. Every year and theatre of the war is covered in detail.

The accounts were given to me by a wonderful group of veterans from some ten countries, with additional material from archived memoirs and letters, oral history recordings, official reports and published books, so that 12 countries are represented: Britain, USA, Germany, Italy, Japan, France, Norway, Poland , Yugoslavia, the Netherlands, the USSR, Greece.

Our World War Two veterans are now all in their eighties and nineties, and it was inevitable that some of them would, as they themselves would say, cross the bar in the years since they told me their stories. I am sad to have heard of the deaths of Earl Beegle (USA), Cdte Sergio Parodi (Italy) and Volkmar König (Germany).  In 2012 the death was announced of Captain George Hunt DSO* DSC.  I never had the good fortune to meet him: he lived in Australia, but we corresponded and occasionally spoke by telephone. Like Volkmar König, he was a good friend to the book. Authors often dedicate their books to someone; I felt that this book could not have been written without the tremendous help of four very special submariners: George (Royal Navy), Volkmar (Kriesmarine), Aart Hopman (Royal Netherlands Navy) and Mario Rossetto Regia Marina), and so the book was dedicated to the four of them, thus conveniently honouring both sides of the the conflict equally.

The great joy of writing this sort of book is meeting, or at least being in contact with, a fantastic group of people. I flew over to the Hague to interview Peter de Jong - who after the war became his country's Prime Minister - and spent a few days in Kiel with Volkmar König and his wife. Late one evening. Volkmar insisted in walking me back to my hotel, though I felt perfectly safe in the centre of Kiel. He insisted I needed his protection from the rapists and murders lying in wait for me. As we walked thought the deserted streets, I asked him where were all those malefactors. "Ah," he said triumphantly:"They know I am with you!" 

I went back to Kiel for the Centenary of the German U-boat service, and had the great privilege of meeting Reinhard Hardegen - who despite being in his 90s walked everyone into the ground. For anyone who thinks the Germans don't have a sense of humour, allow me to disagree. I had a wonderful time in the company of Klaus Mattes  and Alwin Hullmann and their wives, and we never stopped laughing. 
Another lovely pair were Cyril Bowden and George Woodward, who lived not so far from me - it was good to be able to take them their copies of the book in person. A few months after the book came out, my husband and I flew to Milan to have lunch with the enthusiastic group of Italian submariners who had provided some real gems for the book: Sergio Parodi. Dr Ezio Cozzaglio, Carlo Pracchi and Mario Rossetto - not forgetting Cesare Manstretta, the secretary of the veterans association. One thing is for sure: I may not be very good at asking the way to the station in Italian, but I know a siluro from a prua. In 2012, while driving through Italy, my husband and I were able to call in for coffee with the Rossettos and to see with our own eyes that this lovely character was still going strong at 97 - or 98 as he now is.

You don’t need to know one end of a submarine from the other to read this book: the submariners are very good at making you feel one of their tightly knit crew, and there is a very full glossary of technical terms and British/US slang.