The big difficulty in a book about gear is figuring out how to prevent your information from rapidly going out of date. The more you cite specific manufacturers & models, the more likely that new product cycles will outdate your citations within a year or two. These days you could publish in ebook format and issue updates fairly easily, but you would be jumping onto an endless treadmill.

If you aim for a more generalist approach, you might want to emulate the Complete Walker series that ended with IV. I still think that Colin Fletcher's original book is one of the best introductions for beginners, despite every single reference to gear being hopelessly obsolete, in that it gives very clear categories and assists a beginner in organizing their thoughts and approach to the hobby, plus a lot of illustrative anecdotes to give them a sense of what it will feel like to walk into a wilderness wearing a pack to camp there.