CHICAGO — The reclusive author of “To Kill a Mockingbird,” one of the most acclaimed novels of the 20th century, says she never gave her approval to a new memoir that portrays itself as a rare, intimate look into the lives of the writer and her older sister in small-town Alabama.

“Rest assured, as long as I am alive any book purporting to be with my cooperation is a falsehood,” Harper Lee said in a letter released Monday, just as the new book, “The Mockingbird Next Door: Life with Harper Lee” was about to released. The book was written by Marja Mills, a former Chicago Tribune reporter, who moved next door to Lee and her sister, Alice, in 2004 and remained there for 18 months.

Mills responded in a statement, saying that Lee, known to her friends as Nelle, and her sister “were aware I was writing this book and my friendship with both of them continued during and after my time in Monroeville. The stories they shared with me that I recount in the book speak for themselves,”

Mills’ book describes a friendship that blossomed after she first traveled to Alabama in 2001 to write about Lee and Monroeville for the Tribune when “To Kill a Mockingbird” was selected to launch Chicago’s One Book, One Chicago program. She describes her surprise when Alice invited her in to chat and her shock when Lee called her later and visited her the next day at her motel. Lee did not participate in the newspaper story.