Harper Lee, the author of To Kill a Mockingbird, has died at the age of 89.

The book remains a towering presence in American literature, telling the tale of a white lawyer defending a black man accused of rape in the Deep South.

It sold more than 40 million copies worldwide and 55 years after it was published, in 2015, she released the sequel, Go Set a Watchman.

Tributes have been paid to the Alabama-born writer, who rarely gave interviews despite her fame.

Former US President George W Bush, who awarded her the Presidential Medal of Freedom in 2007, said she was "a legendary novelist and lovely lady".

Lee was born 1926 in Monroeville, Alabama. She was the youngest of four children of lawyer Amasa Coleman Lee and Frances Cunningham Finch Lee.

She was a guardedly private person, respected and protected by residents of her town, rarely giving interviews.

Lee's literary agent Andrew Nurnberg said: "Knowing Nelle these past few years has been not just an utter delight but an extraordinary privilege.

"When I saw her just six weeks ago, she was full of life, her mind and mischievous wit as sharp as ever. She was quoting Thomas More and setting me straight on Tudor history. We have lost a great writer, a great friend and a beacon of integrity."

Spencer Madrie, owner of Ol' Curiosities and Book Shoppe, a small, independent book store in Lee's hometown that focuses largely on Lee's works, said: "The world has lost a brilliant mind and a great writer."