The Nazi mystique tends to attract more attention, therefore higher prices.
If you can get a copy of Third Reich Lugers by Jan Still it will be the best. Be prepared for sticker shock. Once his books go out of print, the prices go way up. They will show up on eBay and at military book seller's sites.
Jan's new book is available from him directly. It covers WWI Imperial Lugers as well as most other Central Powers pistols. An excellent book for reference.
http://luger.gunboards.com/topic.asp?TOPIC_ID=13791
Ron