Jack Nightingale is called in to investigate an App that allows a demon from Hell to cause havoc on Earth. Knock Knock is a fast-paced supernatural story about 14,000 words long, featuring his long-suffering assistant Jenny McLean and his nemesis, the demon Proserpine.

Stephen Leather is one of the UK's most successful thriller writers, an ebook and Sunday Times bestseller and author of the critically acclaimed Dan “Spider’ Shepherd series and the Jack Nightingale supernatural detective novels. Before becoming a novelist he was a journalist for more than ten years on newspapers such as The Times, the Daily Mirror, the Glasgow Herald, the Daily Mail and the South China Morning Post in Hong Kong. He is one of the country’s most successful ebook authors and his ebooks have topped the Amazon Kindle charts in the UK and the US. In 2011 alone he sold more than 500,000 eBooks and was voted by The Bookseller magazine as one of the 100 most influential people in the UK publishing world. Born in Manchester, he began writing full time in 1992. His bestsellers have been translated into fifteen languages. He has also written for television shows such as London's Burning, The Knock and the BBC's Murder in Mind series and two of his books, The Stretch and The Bombmaker, were filmed for TV. His book The Chinaman was filmed as The Foreigner starring Jackie Chan and Pierce Brosnan.