We thought the book was a fantastic resource for our first 2 week trip, and when paired with the map we picked up at the airport, it was perfect for our purposes. We kept it in the viser of our truck during the day so we could consult both book and map when choosing our next dive site, and consulted it every morning before leaving for our first dive. Then last year I forgot to bring the book and found we really didn't need it anymore.
That leads me to say that I think its an amazing resource for your first one or two trips to Bonaire (depending on how long you're staying and how much you're diving), but once you've experienced most of the sites, you'll likely find you don't need it anymore.
So if its your first trip, I think buying the book is an excellent idea.
PS If you buy the book, note that when it says "long swim" on many of the southern sites, it simply means there's a sand flat prior to encountering the reef. We never surface swim on Bonaire, instead swim over the sand flat checking out the coral outcroppings and looking for critters in the sand. You don't use much air doing this and we've found some amazing juveniles and large eels doing this. Also, if you swim back over the sand flat at the end of your dive, it acts as your surface interval and is much more interesting than hanging at 15' for 3-5 min.