The information provided here can turn you into a proficient photo detective.
They can also be located in the catalog by using the subject headings: Many old photographs were printed on card stock that had postcard backs and could be mailed.
Although this book deals exclusively with British military photos, it is an exceptionally fine source, worth looking at to learn what kinds of details to watch for, even if your soldier wasn't British.
Although not specifically intended to identify photographs, several books about American military uniforms, headgear, and insignia are sufficiently detailed to be useful for that purpose.
The process for these pictures was done by using a plate, made of copper and using silver iodide to expose the picture, along with exposure to light.
Tin type pictures were introduced in 1853, and became extremely popular.