After the support system has been worked out for your treehouse, it's worth creating the house with a style that you like. This can take some advance planning, as certain elements like deep eaves and window framing, need to be factored into the construction section and can't simply be added at the end like cladding or paint.
Probably the most popular style is the old-fashioned rustic effect using rough boards or cedar shingles. The roof is often steeply pitched with deep eaves and walls are sometimes built off the vertical for an aging appearance. In urban areas it's quite common for treehouse builders to match the design of their main house. These avoids clashing and can keep Home Owner Associations happier.
There are a lot of treehouse books featuring completed treehouses as design inspiration - try Exceptional Treehouses
, Tree Houses
, or New Treehouses of the World
. For other treehouse books covering more of the structural issues, many of Nelson's or Stiles' books
do a good job of covering the details.