I guess I can reiterate much of what's been said:

Anything with specific makes and models of gear will be partially out of date by the time it is published (like trail guidebooks) and definitely out of date in a year or two. It isn't just makes and models but materials that can change in a hurry. 5-6 years ago, most things were silnylon, with spinnaker being the hot lightweight fabric.. Now, although silnylon is live and well, spinnaker is dead and cuben fiber is the hot-selling item. Another example is the current craze for dri-down. The high end manufacturers are, so far, avoiding it, because they feel it needs a lot more testing.

Another issue is, will you include the "cottage" manufacturers? Their products are often better quality, lighter in weight and (unless made of cuben fiber) less expensive than those of the big manufacturers that sell through REI. On the other hand, since many of these are small, one person or one family firms, some of them will not be around 2-3 years down the road, while new firms will have come up with new ideas.

If you are not a really well-known personality, you will find it very hard to compete against the likes of Andrew Skurka, Justin Lichter and even Ryan Jordan. (I'm not an RJ fan.) You might have to do some really adventurous and well-publiized expeditions to get yourself noticed just to sell your book.

Finally, there is a lot of material on the topics you plan to write about on the home page of this site, left hand column. I send beginners there first, to "Sgt. Rock" second and to Mark Verber third.

You might consider concentrating first on skills and techniques rather than on gear. For example, understanding thermoregulation (regulating one's own body temperature) is a critically important skill which is often skipped over or omitted in beginners' books. The same is true for acclimatization to high altitudes. The gear won't do a lot of good without the skills to use it!