'SUMMER afternoon." Henry James considered these two words the most beautiful in the English language. They also conjure up the very essence of childhood.The setting may be urban, suburban, or rural, the family may be wealthy or poor, but no matter - for children, summer is synonymous with outdoors and freedom. School's over, shoes are off, and a seemingly endless stretch of sunny days spool out ahead, rich with promise. Since so much of summer's focus is on the outdoors, it's an ideal time to foster ecological awareness. With a little encouragement, children's natural curiosity about the world around them can blossom into a sense of responsibility for the environment. And what better resource for such encouragement than children's books? Most families with school-age children plan summer vacations. Whatever your destination, and whatever the time frame (day trips count, too) there's sure to be an environmentally geared book that ties in. All it takes is a small investment of time by a parent: a trip to the library or bookstore to search out books on the subject. Here are a few for starters: If you're heading to the ocean this summer, along with swimsuits and sunscreen you might tuck a copy of Michael Foreman's One World (Arcade, $14.95, ages 4 to 7) into the beach bag. In this story, a brother and sister discover a tide pool and painstakingly re-create it in their bucket, only to find that the real tide pool is polluted. What to do? Clean it up, of course. With its gently didactic, positive message - that we are all responsible for our world, and each one can make a start in his or her own backyard - and its light-drenched watercolors, this book is ideal for sparking an early interest in the environment. For the read-aloud set, The Whales' Song (Dial, $14.95, ages 3 to 6), by Dyan Sheldon, is one of the all-around best new books out this season. Young readers will instantly be drawn into the mysterious world of these great sea creatures. Sheldon's poetic prose is illuminated by first-time picture-book artist Gary Blythe's remarkable oil paintings. Reminiscent at times of Monet, they capture the dreamy, magical quality of the story. A different type of whale inhabits How Green Are You? by David Bellamy (Clarkson Potter, $14.95, ages 6 to 9), whimsically illustrated by Penny Dann. Here, a critter dubbed "The Friendly Whale" (with the letters in "whale" representing water, habitat, air, life, and energy) leads young readers through a compendium of useful, hands-on suggestions for conserving, recycling, and protecting the environment. Fun facts abound (did you know that more than one third of all energy is used by people in their hom es?), along with interesting projects - making plant pots or a bird feeder - and brief explanations of things like acid rain and organic farming. If cityscapes are more your family's style and you're planning a trip to an urban hub that has an aquarium, try Do Fishes Get Thirsty? by Dr. Les Kaufman and the staff of the New England Aquarium (Franklin Watts, $13.95, ages 7 to 12). This book, presented in a question-and-answer format, discusses everything from whether fishes are good parents, to those perennial favorites, sharks, to whether sea monsters really exist. Kaufman also talks about the need for public aquariums, and their role in protecting endangered aquatic species. Abundant color photographs accompany the informative text. Going camping? If so, put stargazing on your agenda and check out The Big Dipper, by Franklyn M. Branley (HarperCollins, $13.95, ages 3 to 6). Part of the highly successful "Let's-Read-and-Find-Out Science Book series, it introduces that most recognizable of the constellations, as well as its cohorts, Ursa Major, Ursa Minor, and the North Star. Molly Coxe's cheerful, cartoonlike illustrations are both accurate and entertaining. For older readers, there is the ultimate campfire tale: The Ghost Horse of the Mounties, by sean o huigin (David R. Godine, $14.95, ages 10 and up). While not specifically environmentally oriented, the book brings a remote area - the Northwest Territories - so vividly to life that readers can almost see the vast, windswept plains and a way of life that has disappeared. The gripping narrative poem spins a fantasy based on fact - a violent thunderstorm in June of 1874, during which 250 horses belonging to the original Royal Canadian Mounted Police unit stampeded. All but one were found, and this tale is what o huigin (who spells his name with lowercase letters) imagines happened to that lone horse. Written in free verse, the epic tale - punctuated by Barry Moser's hauntingly beautiful illustrations - builds steadily to its dramatic conclusion.