According to its publisher, Marja Mills's new book about Harper Lee is a chronicle of her friendship with the reclusive author of To Kill a Mockingbird and her older sister, telling of coffees shared at McDonalds and trips to the laundromat in her hometown of Monroeville, Alabama, catfish suppers and feeding the ducks. But Lee, in a strongly-worded letter she released on Monday, says she would "leav[e] town whenever [Mills] headed this way", and "as long as I am alive any book purporting to be with my cooperation is a falsehood".

Mills's The Mockingbird Next Door, which is published in the US this week, details how, in 2004, the Chicago Tribune journalist moved into the house next door to Harper Lee, who is known by her first name, Nelle, and her sister Alice. Mills lived there for 18 months, writes Penguin Press, "with the Lees' blessing". The publisher says that "Nelle shared her love of history, literature, and the Southern way of life with Mills, as well as her keen sense of how journalism should be practised", and "as the sisters decided to let Mills tell their story, Nelle helped make sure she was getting the story – and the South – right. Alice, the keeper of the Lee family history, shared the stories of their family."

Lee first made her objections to the book clear in 2011, when she issued a statement via the Monroeville law firm Barnett, Bugg, Lee and Carter where Alice works, saying that she had "not willingly participated in any book written or to be written by Marja Mills". "Neither have I authorised such a book. Any claims otherwise are false," wrote the Pulitzer prize-winning author at the time.

A new statement, released on Monday in the US and published online in full by Entertainment Weekly, saw Lee write that "Miss Mills befriended my elderly sister, Alice. It did not take long to discover Marja's true mission; another book about Harper Lee. I was hurt, angry and saddened, but not surprised. I immediately cut off all contact with Miss Mills, leaving town whenever she headed this way."

In 2011, Mills's literary agent told the New York Times that the journalist "has the written support of Alice Lee and a lifelong family friend, and prior to Harper Lee's stroke in 2007 she had the verbal support of Harper Lee". Penguin Press, at the time, said that the book was "not a biography [of Harper Lee], but rather Marja Mills's memoir". The publisher has yet to respond to a request for comment on Lee's latest remarks.

Lee wrote on Monday: "I understand that Ms Mills has a statement signed by my elderly sister claiming I cooperated with this book. My sister would have been 1OO years old at the time … After my stroke, I discovered Marja claimed I cooperated with this book … Rest assured, as long as I am alive any book purporting to be with my cooperation is a falsehood."

Mills' book has already drawn a glowing review in the Washington Post, where Heller McAlpin calls it "a zesty account of two women living on their own terms yet always guided by the strong moral compass instilled in them by their father … the model for Atticus Finch in his youngest daughter's first and only novel", and "an atmospheric tale of changing small-town America; of an unlikely, intergenerational friendship between the young author and her elderly subjects; of journalistic integrity; and of grace and fortitude".

Covering why Lee never wrote another novel after To Kill a Mockingbird – "it was hard to live up to the 'impossible expectations' raised by her first, and Nelle hated the publicity and hoopla", writes McAlpin – Mills also details the friendship between Lee and Truman Capote, and their falling out. "Truman was a psychopath, honey. He thought the rules that apply to everybody else didn't apply to him," Lee told Mills, according to the author.

In an extract published by Newsday, Mills writes of a breakfast she had with the author – Lee has "two eggs, over easy … And a side of sausage. And a biscuit" – and of how "I had assumed I would have to keep my distance from the famously private Harper Lee but I couldn't help but enjoy her company. She might have been prickly but she was a delightful companion."

To Kill a Mockingbird won the Pulitzer prize for fiction in 1961, and has sold more than 30m copies around the world. It has recently been released as an ebook for the first time, with Lee commenting at the time that "I'm still old fashioned. I love dusty old books and libraries. This is Mockingbird for a new generation."