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.
|Constructing a 5-String Banjo: A Complete Technical
Guide by Roger Siminoff
Easily the best book on the construction of 5-string banjos.
|Contructing a Bluegrass Mandolin: Mandolin Reference by
(The reason that I purchased both the mandolin and banjo books is that the author describes the two different methods of binding and fretting. In the banjo book he describes the method where you bind first and fit the ends of the frets over the binding. In the mandolin book he describes the method where you fret first, bind second, and then form the binding around the end of the frets. In my observation Gibson typically uses the second method and Stelling uses the first.)
|Guitarmaking: Tradition and Technology: A Complete
Reference for the Design & Construction of the Steel-String Folk Guitar
& the Classical Guitar By Jonathan Natelson and William Cumpiano
A must have book if you intend to build guitars and I'd even recommend it if you don't plan to build but are simply interested in how guitars are constructed.
|Acquired of the Angels: The Lives and Works of Master
Guitar Makers John D'Angelico and James L. D'Aquisto by Paul William
Purchase this book solely for inspiration from two absolute master builders.
|Build Your Own Acoustic Guitar: Complete
Instructions and Full-Size Plans by Jonathan Kinkead
A very good book with some nice ideas for variations on building.
|How to Set Up the Best Sounding Banjo by Roger Siminoff|
|The Luthier's Handbook: A Guide to Building Great Tone in Acoustic Stringed Instruments by Roger Siminoff|
|Understanding Wood by R. Bruce Hoadley|
|Clapton's Guitar: Watching Wayne Henderson Build the
Perfect Instrument by Allen St. John
This book isn't a how-to manual, it's a well written story about master builder (and player), Wayne Henderson.
|Making an Archtop Guitar by Robert Benedetto|
|Make Your Own Electric Guitar by Melvyn Hiscock|