I have just discovered the book I was dreaming about. Of course the marketing team behind Avatar would have pushed for the publication of a Survival Guide to the wonderful and exciting world of Pandora. I mean, they know the market.
You can buy it on Amazon.com for about 11$. They also offer a free peak inside. It's designed to look like a used field guide with crumpled pages and pictures that are stuck in with paper clips and tape. With only 224 pages and I'm guessing lots of big pictures, I wonder how many plants are covered in this book. Oh well, we'll see.
Avatar: A Confidential Report on the Biological and Social History of Pandora (James Cameron's Avatar)