ByJohn SeabrookJohn Seabrook is a free-lance writer living in New York. A lot of novelists, even great ones like Joyce and Faulkner, began their careers as poets. It's a natural development - an intense, youthful lyricism maturing, with experience, into a more balanced, objective vision of life. Philip Larkin is a rarer example of a writer who progressed the other way - from novelist to poet. His poetry reflects his early allegiance to fiction. His syntax, diction, and rhythm are prosy. He has the novelist's empirical mind and the novelist's eye for detail. In fact, Larkin's poetry is often strongest when closest to prose. Tusk/Overlook's reissue of his novel ''Jill'' is a chance for Larkin fans to discover the sources of his art. But approach ''Jill'' as a lepidopterist would a caterpillar - indulge its inelegance. Larkin was, after all, only 21 when he wrote it. ''Jill'' is a Bildungsroman about an insecure young man, John Kemp, in his first term at Oxford University. Two things distinguish it from the Oxford novels of earlier generations, typified by ''Brideshead Revisited.'' John Kemp is from the working class - he eats sausages, not plovers' eggs. And Kemp's Oxford is an industrial, wartime city - not the enchanted playground of Sebastian Flyte and company. But Oxford is still a wealthy world, from which Kemp, shabby and awkward, is excluded. He doesn't protest. Instead he invents a sister, Jill, who is graceful , pretty, and charming - everything Kemp is not. He then meets a real girl, Gillian, whom he imagines to be the avatar of his imaginary Jill. He deceives himself, though, and deception leads to his ruin. Self-deception is a major theme in Larkin's poetry. It implies a need for action, for a shift from ignorance to self-knowledge. Novels deal in actions. When Larkin switched to poetry, he necessarily focused on the condition of self-deception, not the action of it. I love Larkin's poems. But none of them make me feel the misery of self-deception like ''Jill'' (1945) and ''A Girl in Winter'' (1946) do. ''A Girl in Winter,'' published in 1976 by Overlook but now unfortunately out of print, is a better novel than ''Jill.'' The methodical narrative in ''Jill''weighs it down. ''A Girl in Winter'' concerns a single day in the life of Katherine Lind, a sunless wintry day on which time barely creeps by. Embedded in the middle of this day is a flashback to a time six years ago, in the middle of summer. Both winter and summer seem timeless, yet taken together they express some profound change in time. Larkin handles his characters better, too, in his second novel. In general, he has an excellent feel for the Englishness of his characters, but no genius for creating individuals. ''Jill'' suffers because its characters seem stereotyped. But ''A Girl in Winter'' is a more allegorical work. Its characters don't need to be individuals. The emotions and actions of the characters seem determined by the landscape, just as the landscape is expressed through the characters: Larkin's fame rests on his poetry; his novels are read, if at all, as historical documents. Jill is just that. But ''A Girl in Winter'' is a work of art.