My next younger sister went into the Army because she wanted to be a brain surgeon and she received training as an EMT with special background in psychiatric care. She did most of her service in the reserves.
My dad was in the Army, stationed in Alaska during the Korean War, but never actually served in the war.
Several of my uncles served in the army in Vietnam, one driving a truck.
My husband's cousin's husband is also in the Army Special Forces and his wife while they both served.
While I've never been in there service, I've heard their stories about training and watched their behavior change after going into Basic Training etc. I've been on Base at several places.
I started my study of Army practices with a number of field guides:
a) Strategic Intelligence
b) Survival, Evasion, Resistance, Escape
c) Human Intelligence Collector Operations
d) Winter Survival Course Handbook
e) US Army Counterintelligence
f) A Tradecraft Primer
g) Urban Operations
h) Army Field Manual
So what kinds of things do I expect to learn from mere books:
a) names of standard weapons and how they operate and their limits
b) how we defend ourselves via the armed forces
c) what we like to learn about other Armed Forces
d) how a soldier thinks in critical situations
Some other resources I've used in the past include websites to the command structure of the Armed Forces especially post Homeland Security and the titles of officers. Sites that cover the uniforms worn by soldiers of various Armed Forces. What to do in case of biological warfare attack.
Most of what I put together can be reviewed by real soldiers for accuracy but getting the basics down early helps create the sense of believability in a story. What I usually hope to write based on this kind of knowledge is the essence rather than the fact of army life. Most books that really go to the heart of soldier life are written by soldiers or news men that served time in the Armed Forces--most of my stories are about other topics and I only need to give enough accuracy that the supporting officers appear to be real.