The author and publisher of a new book about Harper Lee have stood their ground after the To Kill A Mockingbird author released a statement earlier this week asserting that "as long as I am alive any book purporting to be with my cooperation is a falsehood".

Following Lee's letter, which said that Marja Mills's document of her time living next door to the reclusive novelist and her older sister Alice Lee was written without her say-so, publisher Penguin Press and Mills herself have reaffirmed their positions. Penguin said it was "proud" to publish Mills's The Mockingbird Next Door: Life with Harper Lee, calling the book "a labour of love" and adding that Mills had "done an extraordinary job" telling the story of "the wise and wonderful Lee sisters with readers".

Penguin also provided a letter, printed in full by Entertainment Weekly, from Mills herself, in which Mills writes: "I can only speak to the truth, that Nelle Harper Lee [as the novelist is known] and Alice F Lee were aware I was writing this book and my friendship with both of them continued during and after my time in Monroeville".

Mills also provided a letter from Alice Lee, sent in May 2011, which refers to the 2011 statement from Harper Lee that she had "not willingly participated in any book written or to be written by Marja Mills". Alice Lee's letter to Mills runs: "Poor Nelle Harper can't see and can't hear and will sign anything put before her by anyone in whom she has confidence. Now she has no memory of the incident."

Mills also quoted Harper Lee's "good friend, Tom Butts, who I had the pleasure of getting to know during the course of writing this book and who remains a friend to this day", with Butts saying: "There has been a growing trusting friendship between Miss Mills and Nelle Harper Lee and her sister Alice since she came here to do a story for the Chicago Tribune. The two sisters welcomed Miss Mills to live next door to them as she researched her book. Both sisters, as well as friends and relatives they introduced to Miss Mills, shared stories of their lives and this area for her book. I observed some of this and participated in storytelling. They were pleased that Miss Mills was going to preserve these stories in a book. They wanted to set the record straight in regard to rumours and myths that had circulated about themselves, their family and the novel."

Mills ended by writing: "I am so grateful for my time with the Lee sisters. It was the honour of my life when they both gave me their blessing to write my book."

The comments from Mills and Penguin Press were released in response to Harper Lee's 14 July 2014 letter, in which she wrote of Mills's book: "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."

Lee also said, of Alice Lee's letter: "I understand that Ms Mills has a statement signed by my elderly sister claiming I cooperated with this book. My sister would have been 100 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."

Lee, who is 88, is known for fierce protection of her privacy. Since she released To Kill A Mockingbird in 1960, she has never published another novel, and has almost entirely avoided the press since a 1964 conversation with the author Roy Newquist, in which she said that her success was "like being hit over the head and knocked cold. You see, I never expected any sort of success with Mockingbird. I didn't expect the book to sell in the first place."