That's a problem I'd love to have. I think the best way to keep a site like this going is through donations, but honestly have no idea whether that is feasible or not.
The overall design looks very nice so Google adsense type ads would take away from that.
Have you thought about getting individual trails "sponsored" by a local outfitter? In the info box on the left side you could have Sponsored by Joes Outfitters with a link to their site, and it would match the rest of the text so it wouldn't even be that noticeable.
Someone mentioned affiliate links in a bibliography section. I could see that working because it's not intrusive, but you'd have to be careful not letting your sources be dictated by what's available on Amazon. I like donations because it keeps the financial goals of the site inline with creating the best experience.
I could see sponsorship links working, maybe on the homepage, or maybe in a separate /sponsors page. I'm not completely sure, it's a tricky problem.