America's fragile coastal islands; The Island, by Bill Thomas. New York: W. W. Norton & Co. $29.95.

Rising from the shoals off Virginia's Cape Charles is an island that owes its existence to a shipwreck. It seems a linen-laden cargo vessel met a hull-splintering end in these shallow waters during a storm sometime during Virginia's early Colonial days. In subsequent centuries, sand and shells slowly collected around the grounded vessel, forming what is today a 13-acre chunk of land. This is, of course, a rare exception to how islands came to be scattered along the coast of United States.

In this book, Bill Thomas, a photo-journalist with a bent for nature and wildlife subjects, takes a broad look at the natural history of America's coastal islands. He not only explains how they came to be, but also describes the habitats they possess and the human impact on their development.

A reader is led from the volcanic peaks of the Aleutians to the coral mounds of the Florida Keys, and then north again to the glacial scrapings that form Maine's archipelago. The colorful journey emphasizes the precariousness of the coastal islands. Many of those described are being moved or eroded by the constant action of the surrounding seas.

This fragile quality makes the islands an ideal subject for the natural historian: Change is observable; each landscape becomes a microcosm for exploring the effect of nature's forces. The author offers dramatic examples, such as obliteration of Sharp's Island in Chesapeake Bay. At the turn of the century, this island had a resort hotel with 75 acres of manicured lawn. Today hotel, lawn, and land are gone, the victims of erosion.

Many pages are devoted to color photographs, making this book a prime candidate drawings depicting scenes of island wildlife. Unfortunately, no maps help round out the visual trimmings.

In addition to his evidence of vulnerability, the author offers samples of conservation efforts, such as Maine's voluntary Critical Areas Program, which are helping to preserve these delicate areas.

But in the end, the photographs make the book. The text, while tropical and informative, has a tough time competing for attention when matched up against wildlife close-ups and gl ossy island vistas.