Carlsbad Caverns National Park, located in southeastern New Mexico, was established as a National Park in 1930 and became a World Heritage Site in 1995. It preserves some of the best examples of limestone caves in the world. The story of Carlsbad Caverns National Park is actually the story of two the one park visitors see on the surface and the one they see underground. The pitch-black underground is as different from the bright sun and blue sky of the surface world as the plants and animals of the Chihuahuan Desert are different from the rocks and minerals of the Cavern.
Very good for what it is - a National Parks companion to Carlsbad Caverns. It has some excellent pictures, a history, a section on the ecology of the cave and the surrounding area, and what to expect on visiting.
Another one I bought at the NP gift shop. This one has more historical info than the White Sands book, but it also has fewer good photos… but that may be my bias. I'm not really fond of caves so someone else may really be down with it.