The day before Graham Greene died in 1991, he signed what has since become known as the "deathbed letter". Biographer Norman Sherry composed it carefully to give himself exclusive rights to quote from Greene's works and other unpublished material.

But the strategic placing of a comma by Greene, hours before he died, weakened Sherry's vice-like grip on his work, and allowed other academics to keep discussing it and quoting from it. One of those to have benefited is Richard Greene (no relation), who has edited a book of Graham Greene's letters, spanning the writer's adult life.

Now an English professor at Toronto University, Prof Greene studied 18th-century literature at Christ Church from 1984 to 1990, and frequently returns to Oxford to see "old friends and old books".

He made his first tentative steps into Greeneland almost by accident. Researching a biography of Edith Sitwell, he was having trouble getting access to letters kept at Georgetown University that Sitwell had written to Graham Greene.

"I whinged about this and got a funny message from Greene's son, Francis, who said 'the problem must be Sherry'.

"Norman Sherry has generally done his best to keep other researchers out of Greene's material but in 2003, Francis suggested he would like to see a book of Graham's letters done, and I thought it would be a terrific adventure."

Graham Greene wrote tens of thousands of personal letters, and Prof Greene set himself the task of making a judicious selection to present an engrossing account of the writer's life in his own words.

Many of the letters, all engagingly annotated, are previously unpublished, and the collection reveals the writer in a refreshing new light. Yes, he was an adulterous husband travelling across the globe to trouble-spots in search of fresh copy, but he is also shown as a compassionate man who is loyal to his friends and a devoted, albeit sometimes distant father.

"He was terrifically caring," insists Prof Greene."Think of his concern for Mervyn Peake (when he was suffering Parkinson's Disease) and people locked up in Papa Doc's worst prison - he was making constant inquiries on their behalf."

Prof Greene had already embarked on the exciting task of compiling the selection when he got an unexpected bonus.

"Francis realised that he had stuffed a pile of his father's letters into a hollowed-out book - and then forgotten about them," he said. "There were a good number of letters to Francis, to his wife Vivien, and basic legal documents. It was really important stuff.

"There were letters to his mother from his early years, letters to Vivien from later years and also a few letters to Francis. He was a doting but often absent father."

Prof Greene made a number of trips to Oxford's Bodleian Library to see some of Vivien Greene's letters.

He said: "Staff there were very good to me. They seemed exhausted but they are very committed to helping you."

Prof Greene also enjoyed visiting the writer's old addresses. Greene was a student at Balliol College in the 1920s, shared a house with Vivien in Beaumont Street in the 1940s, and after they separated, Vivien lived for 50 years at Grove House, in Iffley Turn, until her death in 2003.

The married father-of-two confessed that he relished the opportunity of handling Greene's letters, but their literary importance made him nervous and he had no desire to keep any.

"At one point Francis lent me 1,200 pages of letters and I laid them all out on the dining room table. I thought 'this is like the Rosetta Stone - what if the house burns down?'"

Prof Greene confided that the writer's family realised that some letters would not show the novelist in a positive light, but were prepared to give him free rein with his selection because they were confident that Greene would emerge, through his letters, as a good man.

Some very personal letters are featured, including Greene's passionate declarations to Catherine Walston, with whom he enjoyed a lengthy affair.

"I hope other people will share my view of how good the letters are to read," Prof Greene said. "We go first to novels, but there are other wonderful genres to read and a book of letters can be really compelling."

Prof Greene admitted he was not a fan of Norman Sherry's colossal biography of Greene, which took Sherry 29 years to write. He admired Sherry's tenacity and dedication, but felt the third volume was "riddled" with errors, and would "sit on the shelf rather like an inaccurate telephone book". The quietly-spoken academic is hoping his book will stand the test of time because, 16 years after Graham Greene's death, he has enabled the novelist to speak for himself.

Graham Greene: A Life in Letters is published by Little Brown at £20.