Q'S LEGACY by Helene Hanff (Little, Brown: $14.45). Ever since the publication of "84, Charing Cross Road" Helene Hanff has been a cult figure, with all the delights and drawbacks that such status demands. Delights, because it must be pleasant to attract fanatically devoted readers; drawbacks, because those readers make up a limited audience from a publisher's point of view, and their admiration can lead them into taking liberties, such as placing personal phone calls from any point on the globe at any hour simply to express their pleasures and to hear the author's voice. But Hanff shows signs of breaking out of the cult category, and "Q's Legacy" should prove a useful step in that progress, which began with the 1980 reissue of "Underfoot in Show Business" (Harper, 1962) by Little, Brown. Members of the cult will not be disappointed; for in this new book of amusing and unpretentious autobiography, Hanff tells how "84, Charing Cross Road" came to be put together out of letters addressed to a London bookseller asking for titles that she was led to by reading Q, otherwise known as Sir Arthur Thomas Quiller-Couch (1863-1944). Q is chiefly remembered today as the editor of "The Oxford Book of English Verse," but he published volumes of poetry and criticism, including "On the Art of Writing," which Hanff used as her guide when she was unable during the Depression to continue her formal education. Like many before and after her, she was especially taken by Chapter V: "Interlude: On Jargon," and, puzzled at first, soon found that she had hit upon an opinionated and entertaining instructor, given to praise of the plain style. Quiller-Couch is no longer in fashion with the academy. For one reason, he was a generalist; for another, he held to no particular critical orthodoxy; for a third, and perhaps the most fatal, he was an eclectic enthusiast who believed that literature should both entertain and instruct, and probably in that order. Fortunately, Helene Hanff was (and apparently remains) unaware of these damning limitations. Parts of "Q's Legacy" are accounts of the theatrical fortunes of "84" on both sides of the Atlantic and Hanff's reactions to visiting England and seeking out the plaques that mark the one-time dwelling of the writers Q had led her to. One London plaque is mentioned that derives only indirectly from Q, but only readers of "Q's Legacy" can share in its happy discovery. At least one reviewer is willing to join the cult in the hope that it will swell into what publishers like to call "a considerable following."