Central Powers Pistols, by Jan Still, is a bit of a neither-fish-nor-fowl book. It has many more and better quality photos than most heavy-duty reference works, but also has much more detailed information (particularly on markings and serial numbers) than most coffee table volumes. I wasn't really sure what to make of it at first, but on further use I realized that it is pretty well designed as a collector's reference guide. It doesn't try to dig into the details of each gun's design history, but instead focuses on the circumstances of their actual use. For example, the section on the 1910 Bergmann-Bayard notes that Germany overran the Pieper factory and ordered pistol production to continue, but the Belgian proof house was shut down - hence most of the 1910 Bergmanns used by the Germany military did not have proof marks. It's details like this (and attention devoted to lists of recorded serial numbers and associated markings/features) that make this a particularly good resource for folks who are interested more in collecting than in overall history.