Weapon Mounts for Secondary Armament is a nearly 1200-page reference on all manner of machine gun mounting systems for tanks, aircraft,t light vehicles, and ground mounts. The book was commissioned in 1957 by the Detroit Arsenal as a reference for engineers tasked with designing the secondary weapon mounts for new vehicles, and only a few dozen copies were printed. The original manuscript film was located by Dan Shea of Long Mountain Outfitters (now Phoenix Defense) and reprinted in 2007, because it is a massively valuable source of information on a subject that is not normally given close attention.
For any machine gun researcher or collector, this book is clearly a must-have. It focuses on American equipment, but covers everything that was available to the US military form World War One until 1957, which includes a lot of foreign material. Not just standard material, but lots of experimental guns, tanks, vehicles, and mounts as well. I expect it is fully worth the cover price just as a pictorial reference to inter-war experimental US tanks, entirely aside form the machine guns.
As of this filming, copies of the 2007 reprint are still available, for a remarkably low price of $70.