Complex sentence: His magnum opus was his biography of Jean Sibelius, who had been a personal friend of his, and which used a wealth of hitherto unavailable personal material including private letters and diaries, to which he was given unrestricted access.
Split into simpler sentences: His magnum opus was his biography of Jean Sibelius, who had been a personal friend of his. It used a wealth of hitherto unavailable personal material including private letters and diaries, to which he was given unrestricted access.