Where Giants Roam the Earth : DINOTOPIA: A Land Apart From Time, Written and illustrated by James Gurney. (Turner Publishing: $29.95; unpaged, all ages)

Browsing through his local library one afternoon, artist James Gurney claims to have found an old leather-bound sketch-book containing an 1860s account of the “Adventures and Discoveries on a Lost Island” of a father and son named Arthur and Will Denison. It is left to readers of this gorgeously illustrated edition of the Denisons’ journal to decide whether or not they believe in the existence of Dinotopia, a place where dinosaurs never did become extinct but, rather, flourished for centuries in harmony with humans.

In the journal we find out how, after being shipwrecked in rough seas and carried ashore by friendly dolphins, widower Arthur Denison and his young son Will are amazed to find themselves still alive. But a bigger surprise is in store for the two: Exploring the jungle near the beach, they encounter an odd, garishly colored animal shaped somewhat like an iguana but as big as a hog. Defending themselves from what they think is an attack, they chuck a rock at the strange creature; immediately, they are surrounded by a horde of ticked-off giant vertebrates--triceratops, ankylosaurus, brontosaurus--held in check by a young human named Sylvia.

The Denisons have arrived on the lost island of Dinotopia. It turns out that the tiny dinosaur they have--luckily--only slightly injured is a protoceratops multilinguous named Bixone. She is of Dinotopia’s esteemed class of ambassador/translators, and among the few dinosaurs with a command of the English language. (Will and Bix are destined to become good friends; as Arthur writes of the intellectual protoceratops and her fellows: “The capacity for gentle affection among these creatures is astonishing. I now understand Bix’s exaggerated reaction to the minor injury I inflicted, unaccustomed as she is to any deliberate attack.”)

Because of the incredibly choppy seas and dangerous reefs encircling it, Dinotopia has been cut off from other civilizations for centuries. Humans--or “dolphinbacks” as they are known here--arrive from time to time in the same way the Denisons did; and though many still try to puzzle out ways to do it, no one has ever left. In this almost complete isolation, Dinotopia has evolved a highly advanced civilization, of which Will and Arthur undertake an in-depth study.

Their education begins with the vast Hatchery, where dinosaur eggs are laid and develop, tenderly guarded by human caretakers. Gurney’s intricate watercolor pictures show us Dinotopian children keeping a close watch over the big eggs or displaying the new hatchlings to adoring relatives, and playing with toys modeled on their favorite dinosaurs--a wooden rocking tricerotops or a colorfully painted “lambeosaurocycle,” for instance. And in a typically cooperative Dinotopian mode, notes Arthur in his journal, “Over-worked human parents are fortunate to have the help of dinosaur nannies.”

Many more adventures--among Treetop City’s amazing medicinal plants; in the lowlands inhabited by untamed, still-carniverous Tyrannosaurus rex, and high in the frozen reaches of the Forbidden Mountains--follow, far too numerous to detail here. It’s enough to say that Gurney’s suspenseful tale ends happily for all--especially the reader.

Also worth noting for dinosaur fans is Jim Murphy’s picturebook “Dinosaur for a Day” (Scholastic: $15.95; ages 3-8), illustrated by Mark Alan Weatherby; and for older readers, “The Ultimate Dinosaur: Past, Present, Future,” edited by Byron Preiss and Robert Silverberg (Bantam/Spectra: $35; 336 pp.), a mammoth, beautifully illustrated anthology of “speculations” about the giant beasts written by both scientists and fantasists.