One of my favorite books to just flip through at random is Dolf Goldsmith’s The Devil’s Paintbrush: Sir Hiram Maxim’s Gun. Every time I open it, I find another detail of information that I had overlooked or under-appreciated before – it is a wealth of information on the Maxim gun in all its forms. Better yet, the heavily expanded 2002 edition is still available at the cover price (about $80). Among the 200 pages of material added are appendices focusing specifically on the British, German, and Russian Maxims, plus a section on Maxim’s early automatic pistol designs (with a bunch of photos from the Geoffrey Sturgess collection).
The Maxim was hugely influential in machine gun development and warfare in general, and you won’t find a better reference work on it. Copies are available on Amazon, so head over and grab one!