naturalistic writings mostly, to appreciate the outdoors or learn some neat recipes and tricks. john muir, aldo leopold, calvin rutstrum, horace kephart, sigurd olson, these come to mind off the top of my head.
if i've had a really long day of work or lots of stress or something then i bring along some fantasy gamebooks, stuff like fabled lands or fighting fantasy, along with some dice and paper. mythology and folklore is extra comfy too. one of my favorite books is a hardcover on the folk tales of russia. i bring that with me a lot of the time.
sometimes i read history or psychology, especially authors like jung or campbell, who try to link mythology with the human condition. that shit's perfect reading material by a campfire at night.