When I was in Vienna, I splurged on this book. It contain beautiful treehouse by the architects from La Cabane Perchee (www.la-cabane-perchee.com). The book is German, but the architects are based in France. Expensive book, and couple of the houses are not proper treehouses (don’t know how they got in) but well worth it if you have some extra dosh laying around. The book has really beautiful photos and towards the end it has beautiful draft drawings with rough dimensions of each treehouse. One of the remarkable things that this company designs are the spiral stair cases that wrap around the tree.

You can find it on Amazon.de. For other treehouses, see this post.