Designing a series style or range of covers is essentially a form of branding. It could be an author fiction style, a category or subject matter styling, or an imprint styling within the publishing house. The object is to visually portray a range of books as being from the same stable. Author styles (take Graham Greene for example), are continually rebranded to suit market conditions. Shakespeare has been redressed umpteen times to appeal to a wider range of audiences.
If a publisher wishes to reissue a backlist of modern classics, or a new category of academic titles, the series is essentially packaged (or repackaged) to create an essence, a personality, a brand. Branding a series style of covers needs a concept first and foremost, then we begin creating a visually consistent styling through typography, layout, illustration, photography and production. Concept, creativity, control.
Penguin Random House
The Collins Press Transworld