Bucks County, circa 1936-76--in an episodic, smoothly written family novel which, though occasionally affecting and often diverting, never finds the central focus or consistent tone needed for steady involvement. The much-loving, much-hurting family is that of John Howells, N.Y. ad-man and son of a Pennsylvania senator, who marries Jewish Rachel in 1914; and in the book's first third (slightly reminiscent of Oates' morbid Bellefleur), their newspaperman son Nathaniel impregnates and weds teenage Southern-belle Cally--who promptly succumbs to crippling polio, has the first US baby born in an iron lung, and obsessively craves pregnancies. . . while Nat is near-fatally burned (building a holiday bonfire) and soon dies of cancer at 39. This festival of well-bred misfortune--unsentimentally rendered, thanks especially to possessive mother Rachel's acerbic melancholy--exerts a certain fascination. And the section following Nat's death is the book's best: Nat and Cally's eldest child, brusque Julia (""I can't stand love""), shakes off Family by trying a N.Y. acting career--throwing herself at her director-mentor, ignoring come-home pleas from her young brothers, suffering a visit from wheelchair-ed mother Cally. But after this touching, convincing sequence, Shreve's attention roams from character to character over the years, none of them more than sketched-in: ever-devoted, tart-tongued John and Rachel; Cally, in lesbian love with her black nurse-companion; Julia, who has an empty marriage with a British doctor, writes plays, and worries about disturbed son Theo; brother Peter, a disillusioned M.D.; brother Bumpo, a childish druggie who somehow becomes a famed tycoon (with a nuclear-war game); brother Caleb, a faceless architect. And though Shreve seems to have the material here for a deep-feeling novel--which could have focused on Julia or (as in George R. Clay's similar Family Occasions) on the sibling relationships--she loses the emotional grip in the family-saga swirl, resorting to summaries instead of inner action. Diffuse and sometimes artificial, then, but generally skillful and (in the first half especially) curiously engaging.