THE ENCYCLOPEDIA OF MAMMALS, edited by David Macdonald (Facts on File, $45). Every year, there are several new general reference books on animals. The season's pick is this hefty work of more than 900 pages. The entries are arranged by order (carnivores, primates, small herbivores, etc.) and cover thousands of different species. Top-notch color photographs appear on almost every page and are frequently charming, as with a picture of a koala bear lying face-up on a limb to nibble leaves. Although the text is written at an adult level, it and the illustrations are accessible enough to hook a yound reader on natural history for life. THE SEX LIFE OF FLOWERS, by Bastiaan Meeuse and Sean Morris (Facts on File, $19.95). Despite the rather smirking title, this is an excellent introduction to the complex subject of plant pollination. There are birds and bees, of course, but the authors also discuss how wind, water, mammals, ants, and man himself contribute to the propagation of plants. They take examples from all over the world and illustrate them with color photographs of extraordinary clarity. A picture of a mouselike Australian possum reaching its pointed nose into a flower to extract nectar perfectly explains the principle of coevolution, in which different species of plants and animals evolve in tandem to suit each other's needs for food and fertilization. OUR GREEN AND LIVING WORLD: The Wisdom to Save it, by Edward S. Ayensu, Vernon H. Heywood, Grenville L. Lucas and Robert A. DeFilipps (Cambridge University Press/Smithsonian Institution, $24.95). By describing the diversity of plants around the world, their uses and their history, the authors of this thoughtful and attractively illustrated book aim to show our dependence on the plant world and our folly at letting so much of it be destroyed. The epilogue, rather poignantly, is by Indira Gandhi. In it she says, "Human beings can thrive only in a green and living world." WILDLIFE ADVENTURES WITH A CAMERA, By Erwin and Peggy Bauer (Abrams, $45). Can there be a life of more adventure and beauty than that of the wildlife photographer? In more than 40 years of world travel, the Bauers have captured regal mountain goats perched on Pacific Coast promontories, Indian tigers yawning in the shade, and the red uakari, a South American primate that gazes from a tree like a frighteningly human specter of death. In addition to their stunning photographs, the Bauers describe their journeys and the often unusual circumstances in which they were able to take their pictures. THE UNDERSEA PREDATORS, by Carl Roessler (Facts on File, $24.95). Without anyone really noticing, Facts on File has become an outstanding publisher of picture books on nature. In this one, underwater photographer Carl Roessler interprets "predators" somewhat loosely by including corals and other stationary animals, but by almost any other standard it is a stunning work. It is difficult to believe our oceans are inhabited by creatures as beautiful as the coral polyps -- with bright orange flowerlike appendages -- or as hideous as the crocodilefish, which looks like a cruel joke loosed on the undersea world by Jabba the Hut. CLOSE TO NATURE: An Exploration of Nature's Microcosm, photographs by Lennart Nilsson, text by Hans Krook (Pantheon, $14.95). Lennart Nilsson is best known for his amazing photographs of a developing fetus in A Child Is Born, first published in Life magazine nearly 20 years ago. He has now turned his attention to insects and fossil flowers. In microscopic photographs he shows a bee gathering nectar, the compound eye of a mosquito, and the bud of a 78-million-year-old fern. Nilsson's photographs are unfailingly fascinating because they reveal so much that the eye cannot see. OKAVANGO: Sea of Land, Land of Water, photographs by Peter Johnson and Anthony Bannister, text by Creina Bond (St. Martin's, $30). The Okavango is a broad river defining the way of life in much of Botsqana, in southern Africa. It has nutured human life for 100,000 years, and as Johnson's and Bannister's pictures demonstrate, it nurtures a great many other kinds of life as well. We see ibises high-stepping through marshes, boldly colors -- and spikes -- caterpillars devouring a leaf, and herds of elephants, wildebeests, antelopes, and hyenas crossing the grassy plains. The people of the Okavango, both the river-dwellers and the Bushmen still living in grass huts, appear in these pages to be poor but proud in their vast and still unexploited land. THE LAST OF THE WILD HORSES, by Martin Harbury, photographs by Ron Watts (Doubleday, $30). Anyone who cheered at the end of the movie The Misfits will take heart from this lovely oversized book. Harbury and Watts traveled to some of the last native habitats of the world's wild horses -- in China, France, England, Australia, Poland, the American West, and Sable Island, off Canada's eastern coast -- for glimpses of these untamed, noble animals. Their variations are suprrising, from the elegant American mustangs to the primitively stumpy Przewalski horses of Asia that look almost exactly like horses in prehistoric cave paintings in France. Harbury's text shows how the horses have evolved and continue to live, and Watts' photographs are eloquent pleas for preserving the wild horses' lives of splendid isolation.