John Stott was one of the last century's giants of the Christian faith, a household name in evangelical circles, even coming to the attention of Time magazine, who named him one of their 100 most influential people in 2005.

But a less familiar name is that of Frances Whitehead, his personal secretary for more than 50 years and a woman without whom Stott's own ministry would have been greatly hindered.

After years of spiritual emptiness and searching for meaning in life and the world, the then 27-year-old Whitehead had not been a Christian long when she first encountered Stott – but it was an encounter that almost never happened.

She had landed a job at the BBC, based in the Langham Hotel across the road from All Souls Church, where Stott was a young rector.

The church hadn't made much of an impression on her when she wandered in for a lunchtime concert one day, and she determined not to bother going back again.

Instead it was All Souls' sister church a few streets away, St Peter's Vere Street, that brought her along in her faith with its lunchtime talks, and only after some time there, when she discovered that the two churches were linked, did she head back to All Souls and join their Sunday services.

But even then her pivotal role as Stott's "right hand" might never have come about had Stott not sensed where her talents could be best put to use.

Whitehead had it in her mind to go to Bible college, when Stott unexpectedly asked her to become his personal secretary.

Despite some initial hesitation, she accepted and arrived for her first day of work at the rectory at 12 Weymouth Street on 9 April 1956. It was to be the start of a unique partnership that would last until "Uncle John's" death on 27 July 2011.

The story of Frances and that unique partnership is told in Julia Cameron's new book John Stott's Right Hand.

The book offers rare glimpses into the ministry of John Stott from the other side of the podium, through the eyes of one who spent hours in devoted but unglamorous service hammering out speeches and books on a clunky typewriter, running errands, organising hectic schedules, and even sewing new curtains.

It was a life of service that went far beyond the call of duty, and as the other half of John's two-man team, it is little wonder he came to regard her affectionately as "Frances the Omnicompetent".

But the book also reveals the warm humour in their relationship – Stott also liked to call Frances 'Miss Doom' because of her pessimistic side. It also includes a particularly funny recollection from one study assistant of the awkward moment when he was sat between Uncle John and Auntie Frances in the cinema watching Titanic when the now infamous nude scene began to roll.

"I sank deeply into my seat hoping never to have to emerge," he recalled.

Whitehead, now 89, was at All Souls on Sunday morning for the launch of the book.

Asked what it was like working so closely with Uncle John for so many years, she answered with humour: "It was very busy. There was a lot of typing. John Stott wrote 50 books. I typed 50 books."

Explaining why she wrote the book, Cameron said: "Frances was on the staff of the BBC, working for the redoubtable feminist producer Mary Treadgold, when Stott, out of the blue, asked her to become his secretary. Twenty-five years later they would be known around the world as 'Uncle John' and 'Auntie Frances'.

"Stott, who died in 2011, was to say to one his study assistants that he hoped he would die before Frances as he didn't know how he would cope without her. This book adds a measure of completion to the biographies on Stott himself."