"The Hammond Organ: Beauty in the B" by MarkVail (1997) is the only available reference book on HammondOrgans that I know of. Really, it's really not much of a reference book --- more of a "coffee table" book.
A second edition has now been released.
You can 
buy it on-line from Amazon . (Or Barnes & Noble.) ('Help support HammondWiki!' If you use the previous links to order the book, I get a share of the profits.)
The first edition contained numerous factual errors. See MistakesInTheBeautyInTheB for a listing of some of them. It is rumored that at least some of these errors have been corrected in the second edition.
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