In the middle of a senior commander's course in 1970, then 29-year-old Lieutenant-Colonel Winston Choo was summoned to see the Defence Minister, the late Goh Keng Swee, at his office.

What Dr Goh told him struck him like a bolt from the blue.

"I'm going to make you DGS," he said, referring to Director-General Staff, which was the equivalent then of the Chief of Defence Force.

It was the last thing he expected to hear from the minister at that stage of his military career, Mr Choo told The Straits Times.

"It seemed like sheer madness to me! Yes, he did say that after an assessment, it was decided that I was the best candidate. I did not know what to make of it," he added.

The story of how Singapore's longest-serving military chief, now 79, assumed the top job in 1974 is featured in a new book released today. The memoir, titled A Soldier At Heart, is published by Landmark Books and is based on 40 hours of fresh interviews, oral history transcripts, speeches and Mr Choo's writings, said a statement from the local publisher.

The book was written with the help of former civil servants Chua Siew San and Judith d'Silva, who were with the Ministry of Defence.

Asked whether he had agreed with Dr Goh's assessment that he was the best man for the top job, Mr Choo told ST: "There were at least 20 or more others who were senior to me, and in my opinion, would have been suitable for the job.

"In retrospect, among my contemporaries who were formally trained in a military institution to be an officer, I was among the few who had an A-level education. None had a university degree then. I guess that could have been one of the considerations," he added.

But being earmarked for the top post was not all smooth sailing. He faced resistance from other military leaders in the top brass who had later found out about the plan.

Mr Choo rose through the ranks from a bugler in the Singapore Volunteer Corps when he was 17 as a private, to its highest rank of Lieutenant-General when he stepped down as Chief of Defence Force in 1992 after 18 years.

After his retirement from the Singapore Armed Forces (SAF), he served in numerous other roles. Among others, he was High Commissioner to Australia, chairman of the Singapore Red Cross, and Ambassador to Israel, all experiences that he has recounted in the book.

The 280-page book, which took more than 10 months to produce, also touches on his leadership philosophy, his views on the scholarship system in the SAF, his battle with tongue cancer - which he was diagnosed with in 2017 - and the support given by his wife, whom he married in 1966.

It is peppered with anecdotes, such as when he was in the military with current and former Cabinet ministers, including Prime Minister Lee Hsien Loong and former foreign minister George Yeo.

The book was, for a long time, an unlikely endeavour. Mr Choo said he had resisted writing a memoir for years.

It was in May last year that Mr Goh Eck Kheng, founder of Landmark Books, broached the subject with an old friend, Mr Ramachandran Menon, as well as Ms Chua and Ms d'Silva.

"Together they assured me that I had a valuable story to tell and persuaded me to consider producing a memoir. But it was my wife Kate and my children Karina and Warren who sealed the deal when they argued that I owed it to my grandchildren and their children to leave them my legacy so that they may benefit from my story," Mr Choo said.

"So I decided to tell my story in an honest and truthful manner, without any embellishment. That's exactly what I have done in this book."

Mr Goh told ST: "His memoir is by no means an official history, but is valuable in a different way, in that it gives personal, detailed insights and perspectives that official histories don't always have. This is what makes the book important."

Asked if there was anything he had omitted from the book, and if there would be a sequel, Mr Choo said the very nature of his appointments made it difficult to share many things "without incurring the wrath of the security people".

"I have had to be very careful to share my story within the bounds of the security classification. As it stands today, I feel I have said all I want to say and don't intend to venture into a Part 2."

The book is available for $35 (before GST) at major bookshops. It will be formally released on July 16 by Senior Minister and Coordinating Minister for National Security Teo Chee Hean.