NATALYA, GOD'S MESSENGER by Magda Bogin (Scribner's: $21; 275 pp.) The war's over. Rita knows she'll lose her job to a returning vet. An ad catches her eye: "Natalya, God's Messenger, retiring. Established palm-reading practice for sale, $200. No experience necessary." The place is a storefront on East 7th Street. She's intrigued. Rita dons sequined turban, sets out a crystal ball. Opens for business. And lo! She can read palms; she can see the past and the future. Just one problem: How does she tell her nice Jewish immigrant family in Forest Hills? Her intellectual, activist friends, members all of the Communist Party? And here's where Magda Bogin's first novel puts on some heft. Rita's--Natalya II's--chiromantic aberration aside, Bogin's people are real, flawed and fractious: irascible mother Gittel; gentle father Chaim, who adores Gittel to the point where he has his beloved violin muted so he won't disturb her (for her part, Gittel, with undisguised scorn, calls Chaim "Thing"). Pal/lover Leo, brilliant, committed, who's fought with the reds in Spain, drives a cab he calls "La Pasionara" and who splits rather than witness Rita's "degradation;" friend Vera, whose paw tests out as "the first philosophical hand ever to be seen in New York City." There is a problem: Rita, who can read a palm across the room, goes nuts from overload in the subway. There is a dilemma: She foretells Kennedy's assassination but can she do something to forestall it? An intriguing tale, marred by so many tirades against capitalism, so much sympathy for the poor, dear Rosenbergs, that the book too often takes the form of a tract before regaining its legs.