It seems that almost everyone likes treehouses.  Smiles of recognition turn into grins of enthusiasm as more people discover them and dream about making their own private retreats or family play spaces.  And it's nice to remind ourselves that treehouses are built into the oldest and most forgiving, living things on earth.  Also, history records treehouses as being built as deliberate follies, as challenges for arboreal designers, for merrymaking, and for keeping the spirit of fairy tales alive.  But treehouses can also be social places.  We will visit many that were built to entertain, to hang out with friends, or as guest houses.  Trees come in all types. Master treehouse builders Peter and Judy Nelson, with David Larkin, have embarked on yet another treehouse-discovery expedition across America, this time adding the investigation of backyard playhouses to their agenda.  Now, in The Treehouse Book, they reveal their findings, illustrated and described in the most complete volume yet. From casual treeshacks made from discarded lumber to multitiered feats of fancy, they found shelters representing myriad builders--interesting characters ranging from childhood fanatics grown up, to weekend carpenters, to those who want their grandkids to have the best clubhouse on the block. Detailed how-to information, including plans and drawings, is woven with behind-the-scenes tales of each structure's occupants and stunning interior and exterior photographic exploration.