I've been looking forward to this book coming out for awhile. I have "Himalayan Quest," his book of photography, which is quite amazing. I've always gotten the feeling that Viesturs was more into photography than writing so having a co-author will probably make the book a lot better.
There was an excerpt from the book in one of the adventure magazines a couple of months ago (National Geographic Adventure or Men's Journal, I think). Pretty good, but hard to judge out of context.