Among the quickguides and infographs threads on /k/, there are occasionally more worthwhile and heavily detailed manuals posted. Several of these documents provide vast amounts of info on firearms, military equipment and military tactics. Here is a list linking to some of more notable and interesting manuals.
List of worthwhile manualsEdit
http://stevespages.com/page7b.htm: Hundreds, literally hundreds of weapon operation manuals. This is an absolute goldmine of information; if you ever wanted to know how a particular firearm operates, even if it's only semi common, look no further (with the exception of eastern bloc firearms, in which case there is significantly less information). There is also a very good section about firearm optics and accessories like Aimpoint and EO Tech.
https://en.wikipedia.org/wiki/United_States_Army_Field_Manuals: Mainly information on US military tactics.
http://www.fas.org/irp/doddir/army/ranger.pdf: Arguably one of the best books on squad based tactics.