As far as I have been able to determine, for decades there was nothing formally written about the Okinawan tonfa in particular or Okinawan kobudo in general. Starting in 1971 (six years after he came to America) Fumio Demura published a group of books that provided information about sai, nunchakus (two books – basic and advanced), bo (staff), kama (sickle) and tonfa. His tonfa book was published in 1982. He later added a book on eku bo (boat oar). All the books have been recently updated and upgraded. His website is http://www.genbukai-hq.org.
Of interest is that Tiger Claws / Martial Arts Mart is selling a Master Kit of the Da Mo (=Bodhidharma) Cane with a pair of tonfa and a DVD. The link is