The agency will promote the Collins dictionary range, the forthcoming Road Atlas of Britain and The Times World Atlas.
It is the first time the publisher has used an agency for this work, which was previously handled in-house.
Deliberate PR will work with the publisher up until late autumn and will target consumers through the national press.
The agency will handle a media campaign for the dictionary in the run up to children going back to school.
Director Benjamin Webb will head up the account.
HarperCollins recently hired former Disney PR executive Siobhan Kenny as its new director of comms.