It's 117 pages of story and comes with 17 pages of extras, including character development, research notes, sketches and comic creation. There are no differences or additions to the story from the three volumes, except for the correction of three or four minor text errors and a couple of graphical ones that you'd be hard-pressed to notice.
I'd also like to announce The Rainbow Orchid Supplement. The main part of this features complete annotations for the entire story, as well as extra sketches, artwork, and some interviews. It's not quite finalised yet, but I'll post more news when it is. The book doesn't repeat any of the extras from The Complete Rainbow Orchid. I'm publishing the supplement myself, so numbers will be limited, and I'm hoping it will be available late September from this site, but if not it will definitely be available for Thought Bubble in November.