^^
The book itself at least is wholly written by Nintendo themselves as part of the 25th anniversary last year. That makes it an 'official' release and it's content 'canon', if you'd care about that.
Anyway, I take it Dark Horse was licensed to translate and print the book in the west, which would mean both Dark Horse and Nintendo get a piece of the pie. I don't know the details, but they should both be very happy that this book is doing so well (it's surprising me to honest, I'm also happy about it 
!)
EDIT: Also, it seems to be doing decent in Europe too, if you only take Amazon; it's sold out in the UK, on back-order in France and the #2 seller in Germany. Lastly, if you have it, hold on to it nice and good! I see the second-hand price already exceeds retail on eBay and the like. Especially the Collector's Edition which is going to be worth some serious money.
