Book excerpt: National Review has always published letters from readers. In 1965 the magazine decided that certain letters merited different treatment, and William F. Buckley, the editor, began a column called ''Notes & Asides'' in which he personally replied to the most notable and outrageous correspondence. Culled from four decades of the column, Cancel Your Own God dam Subscription includes exchanges with such well-known figures as Ronald Reagan, Richard Nixon, John Kenneth Galbraith, A.M. Rosenthal, Auberon Waugh, Arthur Schlesinger Jr., and many others. There are also hilarious exchanges with ordinary readers, as well as letters from Buckley to various organizations and government agencies. Combative, brilliant, and uproariously funny, Cancel Your Own God dam Subscription represents Buckley at his mischievous best.