Mills & Boon

Mills & Boon is a romance imprint of British publisher Harlequin UK Ltd. It was founded in 1908 by Gerald Rusgrove Mills and Charles Boon as a general publisher. The company moved towards escapist fiction for women in the 1930s. In 1971, the publisher was bought by the Canadian company Harlequin Enterprises, its North American distributor based in Toronto, with whom it had a long informal partnership. The two companies offer a number of imprints that between them account for almost three-quarters of the romance paperbacks published in Britain. Its print books are presently out-numbered and out-sold by the company's e-books, which allowed the publisher to double its output.

From the Wikipedia article “Mills & Boon”, licensed under CC BY-SA 4.0.

An imprint is a brand inside a larger publishing house, often with its own editors and list. Imprints of large groups usually acquire through literary agents; read recent titles to see whether your book fits the list, then target agents who sell to it.