I rarely grow tuberous since they don't do well in the south. Have you tried the ABS book store? It might be a good starting place.

http://begonias.org/

Go to the tab "Shop" and look over what they offer. I do not have any books on begonias but all the issues of the ABS periodicals which you can buy on DVD. You can try the Scottish begonia society since they seem to excel in growing tuberous. The best growers in the states for tuberous are in CA but you need to find those growers and pick their brains.

Report back if you find something that helps you. I think we all would like to find some success in growing these beauties.

There is a full chapter on tuberous-tuberhybrida in 'Begonias, the Complete Reference Guide', by Mildred L. Thompson and Edward J. Thompson. I don't know where you are located but you might check your local library to see if they participate in the 'Interlibrary Loan Program' and can find a copy for you to borrow. There is a chapter there on tuberous and tuberhybrida, very complete with growing advice. Published 1981. A British book, 'Growing Begonias' written by Eric Catterall, supposedly covers every aspect of the care and cultivation of tuberous and non-tuberous, published in '84 by the Timber Press in Oregon. The tuberhybrida are very popular in the U.K.