Golden the Rainbow Gardens Bookshop has the best selection of Hoya books anywhere. There is also a new book that was published by Botanova in Sweden and it is good, my favorite of the two I have.

http://www.freewebs.com/botanova/books.htm

Don't forget the online Stemma Journal!
http://www.stemmajournal.com/

The International Hoya Association
http://www.international-hoya.org/fraterna.html

Mike

I agree with Mike...I also have the hoya book from Botonova from Sweden(english version) "The Genus Hoya". The pictures are to die for!! It gives a brief description of the hoya , picture of the leaves. Its a great reference book when you're looking for some info on a specific hoya. Although there are a few I like that's not in the book. But there are so many varieties, its impossible to have all of them in one book.