I think I've asked this in comments before, but I want to open it up a bit. Kenneth Jackson's Crabgrass Frontier is the best broad/survey history I've ever read of the formation of American cites and suburbs. Unlike a lot of survey's, Jackson actually takes positions and makes arguments. I've read a lot of books on cities since, but none that take a macro level look at the history.
A treehouse can be an ideal office space, vacation rental, and way of reconnecting with your youth.