Did any of you watch the 8 1/2 minute video on the Amazon page. It has the two who started collecting the photographs and how this evolved into the book.
Over the years I've bought a couple other books like this, but what I noticed in the presentation and sale of this book is something different than those others. Other books in their presentations say things suggesting things like "we don't know if the guys in these photos are gay or in past times simply guys who had intimate relationships." As though they are not judging the photographs in any way.
Some of the first books of this type were based on wartime pictures, particularly WWII photos, where the crucible of war may have made men more apt to rely on companionship etc. Books like this one:

There's another one called: Men of WWII: Fighting Men at Ease.
Another called PICTURING MEN where it's written:
Can you imagine that at one time in the not so distant past, men of all walks of life--cowboys, soldiers, athletes, businessmen--felt comfortable enough with each other to display their kinship openly? A hand laid firmly on a shoulder or knee, an arm draped around another man's neck--regardless of sexual orientation, these were demonstrations of genuine affection. Picturing Men starkly contrasts the calm affection displayed in earlier photographs with the absence of intimacy in photos from the mid-1950's on.Some others are:
Dear Friends: American Photographs of Men Together, 1840-1918; David Deitcher
Affectionate Men: A Photographic History of a Century of Male Relationships; Russell Bush
The Invisibles: Vintage Portraits of Love and Pride. Gay Couples in the Early 20th Century; Sebastien Lifshitz
What the guys who put this new book together are saying is that "we know what love looks like and these photos are of men in love." They
are giving a judgment to them. It's just as valid, to me, as saying they could be demonstrations of intimacy without that touchy sex label involved. After all, a picture is worth a thousand words...and those words can be contradictory.