#1 Reef Fish ID, Tropical Pacific by Allen, Steene, Humann, Deloach: This book has some serious volume and plates of the many varieties of fish found throughout pacific. I use it in Australia's GBR, Papua New Guinea's Coral Sea, and Hawaii. There are lots of names for one fish, but the scientific names are generally correct. I like having all the plates readily available. I got mine from Amazon and got it used (didn't even look opened). Only thing I did not like is the soft cover. It is a heavy book though - hard cover would make it worse.
#2 Coral Sea Reef Guide, by Bob Halstead. Bob halstead does a lot of excellent books in PNG and Aussie GBR areas. If you are going to the GBR in Australia, you'll want this to see all the amazing creatures - not only fish. There are a variety of photos and often a bit more info on unusual traits and odd behaviors. I got this book also used on Amazon and the hard cover was slightly dented on one corner (I think I did more dmage to the whole book traveling to and from Australia and on my liveaboard in PNG).