Reliable site for downloading Russian language books

[Der Alte Fritz]

Found a couple of good sites recently
https://armyman.info/ has a good selection of modern books plus some old manuals, some from the 1930s worth a look

This one is more general but does have a range of military books
https://www.litmir.me/bs/?g=g22

[Der Alte Fritz]

http://militarylib.com
got some nice manuals and other titles such as "The art of driving trains in the front line!"
as well as loads of modern books.