I don't mean to discourage you, but there are hundreds actually thousands of books on National parks and other subjects. There is very little money in publishing them, since they do not sell by the millions, and most of them end up being dumped for a big loss. Publishers must sell them to book stores, and its a tough sell.
In this kind of business, credentials are everything. Being published in National Geographic, or having other successful books, Calendars, etc helps, but is no guarantee. I'd work on getting your creds built up, submit to magazines and other publications, a publisher is not likely to even talk to someone who doesn't have some very impressive credentials.
Get a agent, you will get nowhere fast without a good agent.
As you noted, self publish. Perhaps you can get custom orders for calendars for local businesses.
Good luck, but just having good or even surpurb photos might not help much, they are a dime a dozen.