Given that the product is for penguin, Here's some of their series' They all work quite well, using colour and/or typographical choices to unite them thematically. The hardbacks that use different colours and patterns are beautiful and work well as a series because of their rigid layout throughout.
My only real issue is the newer 'Great Ideas' series, that is much loser, tying the books together only with a colour. The images and layout vary so differently that I don't see them working that well as a coherant series. The individual covers are nice, but within my own work, I want the books to be tied together with more thought than this.