The Many Faces of Weimar Cinema: Rediscovering Germany's Filmic Legacy (Screen Cultures: German Film and the Visual Book 5) Kindle Edition

I am almost through reading this book and every essay is interesting and provocative. The book has encouraged me to seek many of the films discussed here and rarely available to the US market. The more I learn, the more astonishing the rise of the Nazis was considering the culture of Weimar.

I highly recommend this reference work. It's easy to read and if you want only one book on films made during the Weimar era, this is the one for you. It's not a general, encyclopedia-type source for information, but the essays here cover a wide range of films and topics such as the star system, multi-language versions, and plenty on Conrad Veidt. Many people are unaware "that what can be legitimately labeled as Expressionistic filmmaking constituted at best a small segment of the filmic output and by no means not the dominant one." There's lots of illustrations, especially for lesser known films, with descriptions that will increase your understanding of them. You can read the preface and an overview here.