The first reference I would highly recommend is membership in the Guild of American Luthiers. They publish an outstanding quarterly journal and offer a wealth of information for luthiers. If you are interested in building instruments or are just want to read about how it is done you should join The Guild.
In addition to membership in The Guild, I highly recommend any and all of the following reference books. While they are not all specifically about banjos they are all great luthier reference books.