Who is Arthur Schlesinger, Jr. from Robert Kennedy and His Times and what is their importance?

Arthur Schlesinger, Jr. is the author of this book, as well as A Thousand Days, memorializing the Presidency of John Kennedy. Schlesinger was a close personal friend and adviser to both John and Robert Kennedy. As such, he became the primary historian of these two political careers, having first-hand and intimate knowledge of both their personal and political lives.