''Northanger Abbey ''is both a perfectly aimed literary parody and a withering satire of the commercial aspects of marriage among the English gentry at the turn of the nineteenth century. But most of all, it is the story of the initiation into life of its naive but sweetly appealing heroine, Catherine Morland, a willing victim of the contemporary craze for Gothic literature who is determined to see herself as the heroine of a dark and thrilling romance. When Catherine is invited to Northanger Abbey, the grand though forbidding ancestral seat of her suitor, Henry Tilney, she finds herself embroiled in a ''real ''drama of misapprehension, mistreatment, and mortification, until common sense and humor--and a crucial clarification of Catherine's financial status--puts all to right. Written in 1798 but not published until after Austen's death in 1817, ''Northanger Abbey ''is characteristically clearheaded and strong, and infinitely subtle in its comedy.