Amazon.com has a terrific list of books in all categories, and particularly one for 'used' bonsai books, many of which are geared to indoor trees, which are automatically going to be tropical, or subtropical. If you use their search buttons/windows to first look under 'used bonsai' and then find ones on 'indoor' trees, you'll see lots at very, very inexpensive prices, with notes on their individual condition. Paul Lesniewicz has a good one if you either want to get it new, or an older edition, and there are many others.

The best book on Ficus is by Jerry Meislik, "Ficus, The Exotic Bonsai". He covers everything you'd want to know about this type of bonsai and most of the material ought to apply to most other tropicals as well.
I highly recommend this book.
Krys

I also have a copy of "Ficus, The Exotic Bonsai" and agree with krys. Most bonsai books repeat the same generalities. A good monography like Meislik's is uncommon and definitively worth buying.