A new erotic fiction novel sold more than 80,000 paperback copies in Britain in just six days last week, says The Week. Sales outstrip the opening-week sales of the E L James mega-hit, Fifty Shades of Grey.

 American author Sylvia Day's Reflected in You tells the sultry story of heroine Eva and the "beautiful and brilliant, jagged and white-hot" Gideon Cross. According to Nielsen BookScan, only two novels – JK Rowling's the Casual Vacancy and Dan Brown's the Lost Symbol - have had stronger first-week sales since records began in 1998.

The erotic novel has been a boon for publisher Penguin, who picked up Day's first book, Bared to You, a month after she self-published it in April this year. With its sequel, Day has produced Penguin's biggest paperback seller, overtaking popular Penguin authors like Jeremy Clarkson and Jamie Oliver.

Philip Stone, The Bookseller's charts editor, told The Guardian: "I think its mega-sale proves two things: one, that the erotica boom is here to stay, and two, that there's still life in the printed book."

Reflected in You went on sale in e-book formats a month ago, and has held top spot in the Amazon.co.uk Kindle chart for almost the whole period since. "Despite this," says Stone, "the paperback edition enjoyed one of the strongest ever opening-week sales for a printed book since records began."

Readers' reviews on Amazon.co.uk of Reflected in You are overwhelmingly enthusiastic, with one reader calling it "an emotional roller coaster," adding: "There is drama and angst, there are secrets and misunderstandings; there are arguments... but then you also have the hot sweaty make-up sex."

The book receives an average of 4.7 stars out of five, while Fifty Shades of Grey, not universally praised for its literary standards, earns only an average of 3.2. But overall sales of Fifty Shades of Grey are still way ahead of its challenger's. Earlier this year, E L James's book became the UK's biggest bestseller of all time, with 5.3 million print and digital copies sold.