Few structures evoke the nostalgia of childhood like the treehouse. As a child, one of my best friends had a treehouse, six feet up the oak tree in her yard, complete with a rope swing and a trapdoor. I lived for afternoons there, sipping lemonade and munching on pretzel sticks within the walls of her hidden kingdom. I knew that treehouse to be a pirate ship, a faraway cabin, a spaceship, and a mansion. For years, authors, designers, and builders David and Jeanie Stiles have been inviting others to revisit the playfulness of their childhoods. With numerous projects and 27 books—including several on treehouses—under their knowledgeable belts, this do-it-yourself couple of East Hampton has made a career out of building backyard escapes and writing woodworking how-tos, in order to get readers outside and enjoying nature. In the past year, the importance of being outdoors has become even more apparent for …