The Collector's Guide to Cloth Headgear of the Allgemeine and Waffen-SS:

This new book provides an organized, chronological guide to the
evolution and development of the myriad types of soft headgear
worn by the SS. As the pre- and war-years progressed, geographical
areas of operation changed, and the composition of the SS divisions
evolved. So also, did the headgear worn by these troops. This reference
illustrates a great number of these examples with over 600 photos
including more than half in color. The overwhelming majority of these
pictures including many rare original candid period snapshots have
never been seen before in any previous publication.