THE RAREST OF THE RARE: Vanishing Animals, Timeless Worlds by Diane Ackerman. (Random House: $23; 174 pp.) Here's a writer whose back yard, the center she writes from, is quite literally the entire planet. From what I can surmise, Ackerman gets a notion, usually to go see something marvelous, and then goes and sees it. In this collection of essays, her burning curiosity takes her to Tern Island (somewhere in the Pacific Ocean) to see the last remaining monk seals, to the Amazon, to "the remote, storm-tossed Japanese island" of Torishima to see some of the last short-tailed albatrosses, to the Mata Alantica, a narrow strip of rain forest on the northeast coast of Brazil to see gold lion tamarins, "the most beautiful monkey in the world," to Southern California to see the monarch butterflies and to the Florida scrublands to think about rare insects and biodiversity. Each of these essays is also a portrait of the scientists who study and advocate these animals and habitats. Ackerman's style is reportorial; there's very little personal reflection. She is dedicated to her subjects in a serious, formal way, even as her inspiration reveals the whimsical, impulsive fascination of scientists. This is not a characteristic I'd associate with scientists.