Over the last forty years, Aaron Lansky has jumped into dumpsters, rummaged around musty basements, and crawled through cramped attics. He did all of this in pursuit of a particular kind of treasure, and he's found plenty. Lansky's treasure was any book written in Yiddish, the language of generations of European Jews. When he started looking for Yiddish books, experts estimated there might be about 70,000 still in existence. Since then, the MacArthur...