Cockfighter is the one for me, love it. Warren oates basically silent for the whole film. Book is great too.
I rewatched the shooting recently but couldn't get into it. I've got ride in the whirlwind and Two-Lane blacktop somewhere tho, should rewatch them. I still remember that really good scene in blacktop with the girl going around the streets scratching together money, it's all shot on a long lens with real people.
There's this big hardback book I've still got, after buying it in Prague when I was 18, called "Cult movies" by Danny Peary. Great write ups of loads of films, and I think Monte Hellman is in there twice, for the shooting and two-lane blacktop. Not sure if there's a single director who could be described as more cult.