Book Series: Design Harmony

I read a lot of series -- both fiction and non-fiction, and I feel that continuity and harmony between book covers in a series is important. They need to stand on their own, yet fit together. Here is one I came across that blew my mind.

Question: What book series have you come across that have a unifying design concept? Are there any that you've come across that not only fit together well conceptually, but literally fit together like puzzle pieces?

Oliver Sacks is a physician, best-selling author, and professor of neurology and psychiatry at Columbia University Medical Center. He is the author of ten books, and was the first to receive honors as a Columbia University Artist, in recognition of his contribution to the arts. Here is the link to his website if you're interested  http://www.oliversacks.com/books/island-of-the-colorblind/