This is a term I learned just yesterday from a buddy of mine. It refers to books like the Executioner, The Butcher, the Penetrator, the Death Merchant, and my favorite, Nick Carter Killmaster.
The stories were full of action sprinkled with some lurid sex scenes. They showcased a protagonist who was more of an anti hero and was always a war vet and had the emotional scars to prove it. What was unique about these books is that even though they were pulpy trash, the mere fact that the characters were Vietnam vets made the stories appropriate for the times and added some social commentary to the work. Far different from their predecessors such as stories from Edgar Rice Burroughs and Doc Savage to name a few.
I would say the most famous series of the genre was Don Pendelton's the Executioner, featuring Mack Bolan, a Vietnam Vet who went around waging war against the Mafia. Later on he would join the Able Team and they battled international terrorists when the series went from being published by Pinnacle Books to Golden Eagle. This is where the series faltered in that I liked it much better when he fought the Mafia. Just a personal preference.
My other favorite, Nick Carter was called the Killmaster. He got a little bit of a sixties face lift and was made more violent. He was famous for saying 'If I am wrong I will apologize,' then proceeded to maim and kill the bad guys.
Other books such as the Butcher, The Penetrator, the Death Merchant etc were knock offs of those two series and didn't have quite the quality or style of either one.
Today it appears this genre is dead. But I see its influence in graphic novels such as Sin City. Quentin Tarrantino borrowed heavily from these books as well. I will pay a huge tribute to them when I write about Huey Dusk's spy years. And you might also see a true men's adventure novel penned by your's truly because I love the genre so much.
I think they are ripe for a comeback in the e book market where they can be sold for 99 cents. Right now you can find them for a $1-2 dollars at used book stores.