George Chinn’s 5-volume opus machine gun-icus is a massive and extremely valuable reference work on the development of machine guns, as well as aircraft machine guns and aircraft cannon. It also includes and entire volume on the actual technical design of self-loading firearms systems, including both the big things like locking and operating systems, but also small things, like trigger mechanisms, extractors, firing pins, and more. For the historian and the design enthusiast alike it is very useful.
Unfortunately, the set is long out of print and both expensive and difficult simply to find for sale. Complete sets can easily cost $1000. Fortunately, the book was published by the US Government and is in the public domain and freely available online. You can find it here or here.