Sidney Sheldon is not one to ignore the vicissitudes of everyday life. Let's say, for example, that you're a bright and beautiful young woman, engaged to marry the scion of an old Philadelphia family when, suddenly, your widowed mother, whom you adore, commits suicide. It isn't, one supposes, beyond the realm of absolute probability that, a few days later, you might be gang-raped in the prison cell where you're serving a 15-year sentence for armed robbery. Fate is funny, after all, and, in more ways than one, it's at its funniest in the genre of fiction to which "If Tomorrow Comes" belongs.

What Sheldon has given us, then, is a lead-and-paste cream puff that's somehow still digestible, a melodrama that turns an avenging angel into a master criminal and doesn't pause for plausibility. There's a direct line here, one senses, going back to Dumas pe re and "The Count of Monte Cristo"; and Tracy Whitney, our ravishing (and ravished) heroine, is simply Edmond Dante s in drag. Yet that's comforting, really: isn't it nice to know that even in schlocky potboilers there's a tradition one can hold on to? "If Tomorrow Comes" doesn't rise to the heights of early Harold Robbins or the later Judith Krantz, but it does plop down lines like, "By God, she's magnificent when she's angry" with the self-confidence born of sincere shallowness.

Tracy Whitney, the "she" in question, has every reason to be angry, for, practically seconds after the story opens and she's pronounced herself "a princess in a fairy tale," she's on her way to "hell." Trying to strike back at the men responsible for her mother's death, she's been outsmarted. Framed and sent to a women's penitentiary where even the tough kowtow, Tracy first despairs, then decides to turn herself into an instrument of revenge. Since this is a novel, however, a special pardon for her turns up in just a few chapters and, from that moment on, Tracy's not a victim anymore.

The lurid, even overly grotesque scenes set in the Louisiana prison where Tracy loses her naivete' in a hurry give Sheldon a chance to show off his research into jailhouse slang, convict routines and the sexual pathology of female inmates. Laid on with a shovel as this data seems, it also represents "reality" as opposed to Romance and thus appears even stranger. One doesn't doubt that Sheldon is telling it like it is, only in this context the awfulness serves merely to emphasize Tracy's defeat and degradation so that her reemergence is more dramatic, more of a relief to the reader. And, of course, more of a spur to revenge. So when the jail door does get thrown open, it's Never-Never Land outside.

Fully two-thirds of the novel is left at this point, plenty of time for Tracy to metamorphose into a queen of thieves who dazzles fellow con artists and insurance investigators alike. Cunning caper follows cunning caper, after she's taken care of her old enemies and decided she likes "living on the cutting edge of danger." To make things a bit more exciting two new nemeses turn up -- one, a charmer who woos her with his wits, the other, an ur-nerd who might have strayed from a slasher movie. But both, in different ways, are eventually bested.

Sheldon obviously had a good time planning the jobs Tracy pulls off, whether it's stealing a Goya from the Prado or a priceless diamond from an Amsterdam display. For reasons less pertinent to the plot, though, he also tosses in a brief Jell-O orgy the silliness of which casts a rainbow glow over the book's more "serious" love scenes. Does it matter, though? Not at all, because in the ways that count at the cash register, Sidney Sheldon's imagination is knock-proof.

When, not if, tomorrow comes, his seventh novel will be a best seller.