Harper Lee, the reclusive author of To Kill A Mockingbird, has sued a literary agent, claiming that he tricked the ageing writer into assigning him copyright on the classic book.

The move marks a rare step into the spotlight for Lee, who is known for keeping a low profile for such a household name, living quietly in a tiny town in the deep south of America and eschewing almost all media requests.

However, in a shock move, 87-year-old Lee has now filed a lawsuit in a Manhattan court alleging that Samuel Pinkus, the son-in-law of Lee's long-time agent, Eugene Winick, tricked Lee into signing over the copyright on the book.

The case claims that Pinkus "engaged in a scheme to dupe" Lee into assigning the copyright without any payment. The ploy is alleged to have taken place in 2007, five years after Winick became ill and Pinkus started diverting some of his clients into his own company. Lee's lawsuit says Pinkus engineered the transfer of Lee's rights to secure himself "irrevocable" interest in the income derived from To Kill A Mockingbird. It adds that he also avoided paying legal obligations that he owed to his father-in-law's company for royalties that Pinkus had allegedly misappropriated.

Lee has been suffering declining health for some years and has trouble with her eyesight and hearing. The case reveals that when she signed the document she was living in an assisted-living facility after suffering a stroke. It says she argues that she has no memory of agreeing to relinquish her rights to the book and signing an agreement that memorialises the purported transfer of income.

"Pinkus knew that Harper Lee was an elderly woman with physical infirmities that made it difficult for her to read and see," Gloria Phares, Lee's lawyer, said in the complaint. The suit also reveals that the copyright was reassigned to Lee last year after she took legal action. Though Pinkus then ceased to be Lee's agent, he was still getting royalties this year, according to the file. So far Pinkus has made no comment on the allegations.

Lee is one of the most renowned names in modern fiction. To Kill A Mockingbird is an esteemed part of the American canon, with its tale of racial injustice in the deep south. Framed around a young girl called Scout, it also features her father, heroic lawyer Atticus Finch, as he seeks to prevent an innocent black man from being convicted of rape.

The book was published in 1960 to wide acclaim. It won a Pulitzer prize and is a mainstay of literature studies in high schools and universities across America. It has sold more than 30m copies worldwide and was also made into a classic 1962 film starring Gregory Peck in the role of Finch.

However, the book remains Lee's only published novel, though it is far from the only reason that she became a major literary figure. Lee had an intensely close relationship with Truman Capote, who was a childhood friend. She helped Capote on projects such as his famous real-life crime exploration In Cold Blood and acted as a sort of muse, researcher and confidante for him.

But, unlike the gregarious and attention-hungry Capote, Lee has always preferred to remain in the shadows of public life. She has spent most of her life living quietly with her older sister in the small town of Monroeville, Alabama. She has rejected almost all interview requests for the past half-century and, despite keeping an apartment in New York City, has not been a presence on America's literary circuit.

An exception has been an obscure essay contest at the University of Alabama in Tuscaloosa. In 2006 the New York Times wrote a story describing how Lee had agreed to become a judge for the contest, which featured work from young schoolchildren. In one of the only interviews Lee has ever given, the writer told the newspaper about how she was always pleased with the way young people took to her book. "They always see new things in it. And the way they relate it to their lives now is really quite incredible," she said.