Some sites that were useful to me and also some books.
First the Books
Treehouses, ISBN 0-395-62949-7 by Peter Nelson
The Treehouse Book, ISBN 0-7893-0411-2 by Peter and Judy Nelson with David Larkin
Treehouses, ISBN 1-890132-85-3 by David Pearson
Tree Houses You Can Actually Build, ISBN 0-395-89273-2 by David and Jeanie Stiles
Home Playgrounds, ISBN 0-920656-62-5 by Merilyn Mohr....you might have to pick this up used ,but it is useful for measurements in swings and depth of mulch kinda stuff that was surprisingly hard to find.
Treehouses.com , Michael Garniers site
Corbins treehouse Lots of interesting links here to start websurfing
Sheenajz on ebay for rescue trolley
Popular Mechanics did a story on treehouses and ours got in