WITH the publication, a few months before his 70th birthday, of the twelfth and last volume of his sequence of novels A Dance to the Music of Time, Anthony Powell completed one of the most ambitious undertakings of postwar British fiction. The series began in 1951 with A Question of Upbringing; now, 25 years later, Hearing Secret Harmonies rounds it off. The critics in Britain crowned Powell's achievement with lavish praise. It was a classic, a masterpiece of ironic comedy, and the literary editor of The Guardian, W. L. Webb, a persistent denigrator of modern British fiction, cast all his usual doubts aside: "Achieved is the glorious work. Buy it now; we won't be able to afford this kind of writing again." Such a welcome showed that Powell had created an almost 19th-century type audience, readers who had followed the work volume by volume and had made it an imaginative annex of their own lives. Some of the reviews discussed the work in terms normally reserved for popular soap operas. The usual tone for these discussions, a judicious mixture of indulgence and knowingness, suggested an in-group making public their idol.