McCaffrey's popular Dragonrider books--now making the leap to hardcover--are built on one of those ""soft"" scientific premises clear to a certain portion of the sci-fi audience: on the planet of Pern, the main business of life is surviving the recurrent attacks of a strange menace called Thread. Over a period of eons the inhabitants--now unable to trace their own off-planet origins--have learned to counter Threadfall with the fire-breathing, telepathic, time-traveling abilities of the native dragons. The White Dragon is an oddly small but battle-worthy specimen called Ruth, ridden by the gallant stripling Jaxom. Together they head off a disastrous war with the hostile ""Oldtimers,"" help establish a beachhead on the vast unexplored Southern Continent, and incidentally cast unexpected light on the mystery of the colony's past. Sappy.