I recently went on a trip to Yakushima and had the great fortune to come across both Mr. Witham's site and his book. Though most (if not all) of the information found in this book can be found on the website, there is definitely something to be said for having a physical "something" that one can carry around and leaf through.
Yakushima: A Yakumonkey Guide is informative without being dry, and is the kind of book one can flip open and immediately start browsing. I found it primarily helpful simply because it gave me a better idea of the things I could do while visiting Yakushima. Not only did it provide information on popular locations, it also offers a decent list of available accommodations, restaurants, transport, and much more. When one doesn't know the language, it's really nice to have a resource in plain, simple English that will guide you around.
My only minor grips with the book are that it's a smidge pricey at 2,300 yen - most travel guides in Japanese offer full color spreads for about 1,000 yen. Also (and this is not really the author's fault), some of the information was a little..off. For example, I hadn't known that one is required to pay a bus fee to go up to the Joumon Sugi tree hiking trail (total 1,700 for a round trip). The book indicates that a fee may be necessary during "busy times", but the info I got at the visitor's center was that the fee was a requirement.
Also, the Toppy fee (one way) was 6,500 (cheaper than indicated in the book), and flight from Kagoshima to Yakushima was 10,000 yen (cheaper than in the book if you reserve more than 4 days in advance).
Still this is nitpicking - I was very happy have this resource in hand during my trip and would certainly recommend it to someone visiting there. It is especially invaluable if you don't speak the language.
Oh, and by the way, I do NOT recommend biking around the island. A rental bike for a short tour might be ok, but I tried the "around the island" thing and unless you're a hardcore cyclist, you will end up in much pain..