Lots of books have pages and pages of details on building and finishing room boxes. Now that is great if you want to go that route. Many times I just get the box and want to decorate it. This is the book for you.
The Dollhouse Decorator by Vivienne Boulton (ISBN 1-54458-077-6)
I like my room boxes to be decorated like my real life house. I want them to have the things that a normal room would have. These pages remind you of what makes a home.