Hi there!

If you're not familiar with Stemma (a pdf-magazine), I recommend that you get those at the "other" hoya forum. Great resource with lots of information and well written. Unfortunately it's discontinued, but there are quite a few issues available for download anyway.

In Stemma 3:1 there is a review of a book titled "The Genus Hoya". It covers about 120 species of Hoya.

If you speak German I would recommend "Hoya" by Surisa Somadee and Jens Kühne. It covers about 200 species.

The books by Dale Kloppenburg and Ann Wayman I don't have any personal experience of.

Kami

I'll check these out, thanks Kami. I do speak some conversational German, but not enough to read a book, yet alone a specialized book like this.