More than 330 million people visited a national park last year. And many more are headed to one this summer. Environmentalist Terry Tempest Williams joins us to talk about how we can better care for these natural wonders, which she writes about in “The Hour of Land: A Personal Topography of America’s National Parks” (Picador). She speaks tonight at 7:30 as part of DMA Arts and Letters Live! at the Dallas Museum of Art.