These covers are so beautiful! n+m was the abbreviation for Naturwissenschaft und Medizin, or in English, Natural Science and Medicine. The covers were designed by Erwin Poell in the mid 60’s till the early 70’s.
Head over to Things To Look At for a ton more images of his work, all of which are totally amazing.
Also check out Ekusupo’s flickr for even more awesome covers!