Information sources don't always have to be treehouse related to help with your project. The structural design of the supports is the main thing separating a treehouse from other forms of housing, so keep your eyes open for different sources of ideas that you can use.
Interior design magazines can be useful to learn new styles and decorating tips. These can be used to improve the space inside the treehouse and in some cases define the exterior look of the treehouse. Look for accessories and materials that could be re-purposed for use inside a treehouse, where space is tight.