Marvelously funny collection of sketches; What I Know So Far, by Gordon Lish. New York: A William Abrahams Book/Holt, Rinehart & Winston. 162 pp. $14.95.

Of the world as it exists, it is impossible to be enough afraid. THIS epigraph by T.W. Adorno captures the biting, self-depreciating humor of Gordon Lish's collection of fictional sketches. In these 18 pieces of interior monologues, characters - mostly in the voices of stereotypical Jewish-American comic personae - speak of quixotic ambitions, marital obsessions, family deceptions, fickle fate, and the cruel irony of being socially isolated by success in gentile society.

Some of these tales suggest the image of characters whom we know only through their plight and manner of describing it. In ''Everything I Know,'' for instance , a husband and wife report a break-in and assault to a nameless narrator. The couple's account is interspersed with interjections which completely overshadow the subject of their frantic experience.

''How to Write a Poem'' and ''How to Write a Novel'' are satirical disquisitions on the frustrations of beginning to begin a creative act. The narrator's soliloquies reveal endless distractions, causes for procrastination, and writer's block. Other stories evoke the comical, repellent tones of modern Jewish satirists such as Woody Allen.

The longest story is perhaps the funniest. ''For Jerome, With Love and Kisses'' is about the retired or widowed parents of famous American Jewish writers. These parents have ghettoized themselves in an apartment house in south Florida. There, Murray Mailer, Burt Bellow, Gus Krantz, Dora Robbins, the Malamuds, and the Roths, Hellers, Segals, and Allens gossip and promote the glory of their distinguished literary progeny.

Mr. Lish is a marvelously funny writer, whose book should have wide appeal.