The Perils of Little Laura : LIGHTNING <i> by Dean R. Koontz (G. P. Putnam’s Sons: $18.95; 351 pp.) </i>

In two decades, Dean R. Koontz has penned more than 50 novels under a variety of pseudonyms, eight of them in one particularly productive year. Now, however, after two annual back-to-back best sellers, “Strangers” and “Watchers,” he appears to be settling for a little less prolificity. But old habits die hard, which may be why his newest tale reads like two nearly distinct yarns.

In the first and more engrossing section of “Lightning,” a little orphan named Laura bounces from institution to foster home, being saved from an assortment of perils, including two separate molesters by a mysterious “guardian angel.” Koontz is particularly skilled at setting up believable characters and situations.

This proficiency comes in handy because he needs all the credibility he can muster for the second half of the story, when Laura, now a wealthy pop novelist-wife-mother, discovers that her protector is a time traveler.

He has been misusing the invention that creates a “lightning road” through time to smooth out Laura’s bumpy path to happiness. I say “misuse,” because he comes from an era of infamy, ruled by a ruthless dictator, and the lightning road was paved with bad intentions--world domination, for example.

Koontz plays one truly surprising trick with his time travel device, but he also gets so involved in its rules and ramifications that he allows his people to turn into puppets. As children, Laura and her best friend Thelma are charming and human. As adults, seeking to escape the evil era hoppers, they, and everyone else, lose dimension and incongruously engage in conversations you might hear on one of TV’s new warm sitcoms.

Koontz whips up a fair amount of suspense as he speedballs to his finale, but, with all its thunder, “Lightning” never quite lives up to the promise of the early chapters in Laura’s life.