The story of Africa’s greatest modern statesman, Nelson Mandela, will gain another chapter this autumn, with the publication of Dare Not Linger.

Following on from Long Walk to Freedom, the inspiring account of his early life and time in prison that was made into a feature film starring Idris Elba, the book will chart Mandela’s time as South Africa’s first black president from 1994 to 1999.

Mandela began working on the manuscript at the end of his presidency, but the first draft was still unfinished on his death in 2013. Fragments from the manuscript featured alongside snippets from diaries, calendars and letters in 2010’s Conversations With Myself. But now the South African poet and novelist Mandla Langa has finished the task, knitting Mandela’s account together with archival material into one coherent narrative.

Speaking to the Guardian by telephone from Johannesburg, Langa said that at first he found the project daunting, but his guiding principle was to prevent Mandela’s words from “being overwhelmed by my own interventions”.

“I was really aware of the weight of the kind of person he was and the need for his voice to shine through as much as possible in the writing.”

Dare Not Linger is constructed from Mandela’s own words, Langa continued, linked together with additional context. “He had written something like 70,000 words, which in other situations would qualify as a fully-fledged manuscript. But there was still a lot more he wanted to write.”

According to Georgina Morley, Langa’s editor at Macmillan, the book is a remarkable first-hand insight into Mandela’s presidency. “It’s the closest that you will ever get to a true autobiographical sequel to Long Walk to Freedom,” Morley said.

Nothing the former president wrote himself has been reworked, she continued, with the structure of his uncompleted draft preserved, but Langa has returned to contemporary sources to explore the wider picture for readers 20 years on. “Mandela took notes of every meeting he attended, in his own hand. They’re all there in the archive, so Mandla’s drawn on all that to flesh out the narrative.”

Opening in 1993, as the ANC formed a government in waiting, Langa follows Mandela through the 1994 elections—the first in South Africa at which citizens of all races could take part—and into government, laying bare the challenges of making the transition to democracy after five decades of apartheid.

Dare Not Linger is scheduled for October publication in the UK and the US, with a foreword from his third wife, Graça Machel. Translation rights have already been sold across Europe. According to the book’s agent, Jonny Geller, it will play a crucial role in understanding Mandela’s legacy by offering an insight into how power is used.