This is a big book... a really big book! It weighs in at over 1,000 pages and in my opinion, is the most comprehensive book on the South Pacific. It is definitely geared more toward the island hopper and/or complete South Pacific traveler, as there are individual books on the different islands available.
I'm not sure how this title relates to the Lonely Planet South Pacific (I'd consider these two as the leading authoritative sources), but I chose Moon over Lonely Planet because (1) there are 250+ more pages in Moon, and (2) this guide has been out from Moon for 8 editions, since 1979, compared to 2 editions for Lonely Planet. I haven't been let down by this book.
As expected, there are two substantially large chapters devoted to French Polynesia and Fiji. The 13 other islands are also covered in great detail (for example, about 80 pages dedicated to Vanuatu!)
Whereas I consider Lonely Planet guides to be more for the frugal traveler or backpacker, Moon covers a wide variety of accommodations, anywhere from $250 and night and up, down to your basic bungalow. This is helpful because let's face it, in some places I want to splurge for the overwater bungalow! I'd also comment that Moon is a great choice for the active traveler. There are tremendous sections on how to find kayaking, scuba, and surfing tours.
For most people, a guide on a single country or island is going to suffice. However, if you are planning or dreaming of a trip to multiple islands across the South Pacific, I'd take this guide as it has certainly been around the longest and is the most comprehensive choice out there.