For many adults, tree houses conjure only fond childhood memories of secret clubs and the movie "The Swiss Family Robinson." But for some, according to Seth Tibbots, tree houses are home. Tibbots, a tree house dweller, is currently researching a book on tree houses of the world with Laura Ewig and Marsha Earbarrentine. He estimates that as many as 400 "serious" tree houses with electricity and plumbing are in the United States. This count does not include the crude platforms made by adolescents. It only considers multilevel structures with between 1,000 and 3,500 square feet of living area. The tree house pictured here, for instance, is as typical as this idiosyncratic form of housing can be. It has about 1,000 square feet