The 1930s witnessed a major shift in the firm's direction which reflected changes in the marketplace. As library sales increased between the wars, fiction displaced the educational and general lists. At the same time, Mills and Boon specialized in its most successful type of novel, the romance. Romantic novels had already been a strong feature of its lists throughout the 1920s. (266-267)
Although before 1950 many publishers, including Collins, Hutchison, Herbert Jenkins, and Wright and Brown, published a romantic list, Mills and Boon's specialization in this genre was unique. (269)