Complex sentence: He is best known as the author of the definitive biography of his father-in-law Sir Walter Scott, which has been called the second most admirable in the English language, after Boswell's "Life of Johnson".
Split into simpler sentences: He is best known as the author of the definitive biography of Sir Walter Scott. This biography has been called the second most admirable in the English language, after Boswell's "Life of Johnson".