The publisher has also launched a free app, HarperCollins Reader, which will allow readers to search the publisher's available e-book library and download titles.
Accenture, the consultancy and technology firm, will operate the platform, selling and distributing e-books globally. The sites are currently available in the UK, US, Canada, Australia, New Zealand and South Africa.
HarperCollins said the new platform would help the publisher reach new audiences.
Chantal Restivo-Alessi, chief digital officer at HarperCollins, said: "The publishing industry is undergoing a technological transformation unparalleled in our 200-year history. Launching a platform that allows HarperCollins to establish a direct to consumer marketing and sales proposition to expand some of its strongest brands to new audiences means we honour both our past and our present."
Thomas Grayson, m.d. of Accenture Digital Services, said the sites would give HarperCollins "a unique edge" in the digital market.
He said: "Combining Accenture’s suite of digital delivery and analytics capabilities with HarperCollins’ traditional strengths of content acquisition, editing, marketing and distribution, will position HarperCollins as a digital innovator with the tools to evolve its digital strategy. HarperCollins’ increased commitment to the digital value chain gives it aunique edge in attracting writing talent and driving growth in an expanding e-publishing market."
In a statement, Accenture said that HarperCollins would now be able to analyse all online purchase data, leading to greater decision-making capabilities.
It added: "This e-commerce approach will be extended in time to other HarperCollins online properties."