At the bookstore where I worked for over 20 years, I rang up hundreds of couples. With the traditional middle-aged male/female couple, I noticed that men often bought books on war and women bought romances. Of course, this is a generalization because many traditional couples bought all kinds of different books. But, in general, older men usually like war-related books. I've been mulling over why this is so and here are some of my ideas.
Men born in the late forties and fifties were inundated with war stories for most of their lives. Either their fathers or a friend's father probably fought in WWII and Korea. They heard stories directly from soldiers about war and it's consequences. Also, television took over from the radio and covered every aspect of past wars and current ones. Korea, Vietnam, and more filled the airwaves and minds of adolescents and teens. I remember watching endless war movies and imagined myself as John Wayne fighting for our country. Young girls weren't sold this kind of story. They never developed the need to imagine themselves in war. Probably because they were always depicted as nurses or wives in war films/books.
Endless books were written about war in the fifties and sixties, not to mention pulp magazines. I would often play "war" with the local kids on my block until the sun went down and our mothers called to us from the front door of our house. No girls ever participated in these games. It was an unspoken rule.
War attracts young men because they want to prove themselves worthy and heroic. The average age of the WWII soldier was 26; the average age for the Vietnam soldier was 19. Young men flock to enlist because they want their families (and their father) to see them as true men. They only way (as accepted wisdom goes) a young man can prove himself is to face death and survive.
By reading war books (both non-fiction and fiction) men in their later years can imagine themselves at war but without the threat of death. They want to vicariously experience war through books in order to have that feeling of being a true man. At least that's the traditional view of manliness.
I've had many conversations with older/middle-aged men about war books while ringing them up. We share favorite books and recommend new ones. It's a way older men reinforce our shared interests in war. At least in a war that is righteous (WWII, for example). And knowledge of the war (any war) can replace the lack of someone's never having fought in a war themselves.
Younger men have little interest in war perhaps because war coverage has become so anti-war. Certainly, wars in the Middle East were seen by young men as a very bad idea. Modern youth see war in a much different way than older people.
The publishing industry cranks out war-related books every year with their primary market being older white men. And these are the books that I sold over the counter every day to men. And I don't see this changing for a long time.