It is sometimes thought that the renowned essayist Isaiah Berlin (1909-1997) was incapable of writing a big book. But in fact he developed some of his most important essays--including "Two Concepts of Liberty" and "Historical Inevitability"--from a book-length manuscript that he intended to publish but later set aside. Published here for the first time, Political Ideas in the Romantic Age is the only book in which Berlin lays out in one continuous...