NEW YORK (AFP) - Harper Lee's second novel Go Set A Watchman topped the United States bestseller list last year, selling 1.6 million copies more than half a century after her Pulitzer-winning first book was published.

The figures were based on US print unit sales reported to Nielsen since the novel was published in July, snapped up by avid fans who for decades thought the reclusive author of To Kill A Mockingbird would never publish another word.

Pre-orders turned it into an instant bestseller at many retailers the moment its release was announced by publishers HarperCollins last February.

The novel features many of the characters of Lee's 1960 masterpiece, which was turned into an Oscar-winning movie starring Gregory Peck.

Considered a 20th-century classic that defined racial injustice in the Depression-era South of the United States, Mockingbird is standard reading in classrooms across the world. Lee's second published novel, which earned more mixed reviews, was seemingly discovered in her safe deposit box in still-unclear circumstances.

Second and third bestsellers were the latest instalments in Jeff Kinney's Diary Of Wimpy Kid series and British author E.L. James' mommy-porn erotica Grey: Fifty Shades Of Grey As Told By Christian.

Each sold 1.48 million and 1.4 million copies respectively in the US, according to data compiled by Nielsen.

In fourth place was nail-biting thriller The Girl On The Train written by Paula Hawkins, a former journalist based in London. The book topped the list of top 10 Kindle bestsellers on Amazon in 2015, on which Go Set A Watchman came sixth.