William Morrow's line of digital-first mystery titles, Witness Impulse, has broken a sales milestone: its books have sold over 1 million copies in the U.S. The line was announced by Morrow parent HarperCollins in April 2013 and debuted in the fall of that year.

The 18-month-old imprint has "far surpassed our expectations," said Morrow's v-p and executive editor, Dan Mallory. Noting that while Witness Impulse has been releasing print editions of its titles, Mallory said the success of an imprint focused on e-books "represents a paradigm shift in what is possible with digital first publishing.”

Among some of the hits Witness Impulse has published are Brian McGilloway's Little Girl Lost (February, 2014), and Eric Rickstad's thriller The Silent Girls (November, 2014). Both books are priced at $2.99; McGilloway's novel spent six weeks on the New York Time's combined print end e-book list, while Rickstad's novel spent three weeks on the USA Today bestseller list.

Morrow credits the success of these books, in part, to their low price point. The publisher also said that, without having these books tied to a specific print publication date, it was able to take advantage of more retail promotions throughout the year. Morrow's senior v-p and publisher, Liate Stehlik, said she sees digitally-first as "a vital component in strategically growing new authors, and extending the visibility of our existing author base."

Witness First releases titles by a mix of new and established authors. The imprint, Morrow added, has also worked as a launching pad for previously self-published bestellers, as well as newly digitized backlist titles. Witness Impulse is part of Morrow's Impulse publishing program, which started in 2011 with the romance line, Avon Impulse.