The Sunday Times said Hancock's main motivation was to correct the tone set by an unofficial biography published three years before the actor's death from cancer in February.
A bidding war between HarperCollins, Transworld and Random House is said to have pushed the fee up to £900,000 and it could break the £1m barrier, a record for a British actor's biography.
Hancock will meet publishers this week for further negotiations, the newspaper said.
"What I don't want to be seen doing is using my husband to push my own life forward," the Sunday Times quoted Hancock as saying.
"I also don't want a luvvie biography."
Thaw was reported to have been upset by a 1998 biography which focussed on the actor's supposedly troubled relationship with his mother.
Hancock added: "John found it so offensive. It was so hurtful to him and many of his family."
Last episode
A biography is likely to be popular, with Thaw still held in affectionate regard for his performances in Inspector Morse as the opera-loving Oxford sleuth.
Repeats of the show still get good viewing figures, while the last episode in November 2000 was watched by 13 million.
There has been a trend among publishers for wives writing intimate biographies about their husbands such as Pamela Stephenson's hard-hitting account of Billy Connolly's wife, the newspaper said.
Hancock was Thaw's second wife, and they stayed together for 28 years despite a brief separation in the 1980s.