It may be hard to credit in this age of monster McMansions that the “tiny house movement” which began in the hippie `60s, continues to grow. But so say architectural designer and illustrator David Stiles and his wife, Jeanie Stiles, a writer and photographer. Their attractive newest how-to book, Building Small , their 25th, pitches barns, cabins and sheds as weekend and year-round retreats, whether off the grid or on. Would you believe an adorable 8’ X 11’ backyard Tudor or a two-level Japanese treehouse, interior trunk and all? When they’re not in New York, the Stiles live in a barn they renovated in East Hampton and they practice – and demonstrate – what they preach. A few years ago on the Today show, they built a treehouse during a single live three-hour broadcast from Rockefeller Center. Even if you don’t want to follow David’s lovely ink-drawn plans, the book’s space-saving practical tips, alone, are worth attention. Who knew – I didn’t – that screws are better than nails because