We have here a light and harmless book that has appeared on American bookshelves thanks to the ripple effect of perestroika on Soviet-American political and cultural relations. "Soviet Humour" is a selection of caricatures published in recent years in the celebrated Soviet satirical magazine Krokodil. At the grand old age of 67, Krokodil is the oldest magazine of its kind in the Soviet Union; published every 10 days, it enjoys the largest circulation of any periodical--5.3 million copies.In his warm greetings, aimed at the American reader and titled "Let"s Laugh Together," Aleksey Pyanov, editor-in-chief of Krokodil, reveals how this book came to be. The idea was proposed by Herbert Cummings, president of the Workshop Library on World Humour, and James Boren, president of the International Assn. of Professional Bureaucrats.