An entire book full of book covers!
Though Mills and Boon and Harlequin are best known for their romances, it would appear that they published other genres--at least in their earlier days. From 1950:
Though of course the romances do predominate in every era--as shown by this couple in a steamy clinch in 1976:
There's a fair amount of social history to be gleaned from these covers--from changing standards of what what was acceptable (the covers get steadily raunchier from the 1960s on) to changing fashions in clothes, hair and makeup. (Could the couple above belong to any decade other than the seventies?)