The American Wilderness series was published in the mid-70's. I can honestly say that I have read all of these, my orthodontist had a set in his office and that was what I read while I waited for the privaledge of being seen.
This is a pretty nice set, and it has been very popular, but that popularity might work against you if you try to sell them. This set is one of the lowest valued sets in the used book world; and I think it's just because of the sheer number of them that were printed. You can often see entire sets on e-Bay for less than $10 for all 27 (though shipping will cost you a bit). Also, watch out; the World's Wild Places set looks exactly like these books, so they are easy to confuse.
Note, also, I've never been able to find ISBN numbers for these books. Since there are no ISBN numbers on these, I listed the Library of Congress Control Number (LCCN) to help you try to find them.
In Europe, this series was published with The World's Wild Places all as one set with a few title changes; Wild Alaska became simply Alaska, The Bayous became The Bayous of Louisianna, Baja California became Mexico's Baja California, The Sierra Madre became Mexico's Sierra Madre and The Great Divide became The Rockies.