GIANTS, illustrated by Julek Heller, Carolyn Scrace and Juan Wijingaard. Devised by David Larkin. Text by Sarah Teal (Abrams, $15.95). It is difficult not to believe in giants, so common are the myths and legends about them in every culture. But even with all the evidence of their existence, insists this beautifully illustrated and delightfully tongue-in-cheek book, no one has seriously examined the lives lived by the early giants or how they are managing in the modern world. In addition to presenting the history of outsized man, Giants provides such interesting "information" as this: "Giants avoid downhill runs as their momentum is too great. They have to crouch in anticipation of loss of control." THE FLIGHT OF DRAGONS, by Peter Dickinson; illustrated by Wayne Anderson (Harper and Row, $17.50). Equally whimsical, but better written, is Peter Dickinson's exposition on the existence of dragons. Much of the author's "evidence" comes from fiction, both ancient and modern, and the text is liberally sprinkled with quotations from literature to bolster his contention that dragons really could fly, breathe fire and had magical powers in their blood. Wayne Anderson's strange and shimmering illustrations complement the text, encouraging the reader to agree, as Dickinson would have it, that "the dragons live. Inside us." DRAGONS, by Peter Hogarth with Val Cleary (Viking Studio, $16.95). This book provides more information on dragon lore than Dickinson's but it is not nearly as charming. The authors' purpose is to trace the history of dragons from antiquity to the present. The illustrations show images of dragons as they have appeared in art throughout history, all attesting to the power of dragons to endure as archetypes in the human mind.