I've seen a copy of this book:
http://www.amazon.com/exec/obidos/t...f=sr_1_1/104-2272687-1015967?v=glance&s=books
It was very good, but a bit biased to what you'd see in the ocean vs. what you'd see in your LFS or on Live Rock. But, an entertaining and very interesting book nonetheless. (note that its caribbean, and doesn't cover Pacific species)
If that link is busted, here's some more info:
Marine Plants of the Caribbean: A Field Guide from Florida to Brazil
by Diane Scullion Littler, Mark M. Littler, Katina E. Bucher, Jam Norris, James N. Norris
Paperback: 272 pages ; Dimensions (in inches): 0.76 x 7.62 x 4.99
Publisher: Smithsonian Institution Press; (September 1992)
ISBN: 0874746078
Descriptive text and color illustrations for common species likely to be encountered, along with those that are locally abundanet or might attract attention because of some unusual characteristic. Annotation copyright Book News, Inc. Portland, Or.