James Boswell.  The Life of Samuel Johnson.

Boswell wrote one of the most famous and popular biographies in the English language.  Because he was much younger than Johnson (He did not meet until he was 23 and Johnson was 54.), he did not have first-hand knowledge of Johnson's early life, and he only spent a limited amount of time with Johnson after they did meet.  Also, there is only a limited amount of material available for Johnson's early life.  On the other hand, there was a vast amount of material available for Johnson's later life.  There was material from Boswell's personal dealings with Johnson, and material available from other sources. 

Boswell has come under criticism for idealizing Johnson--he cleaned up some of the material he put into the biography.  Also, the biography tends to deal with lively descriptions of individual scenes from Johnson's life and be skimpy on the periods in between those scenes. 

Nevertheless, Boswell's biography is a landmark in the developing art of writing biographies and has influenced the discipline of biography writing as well as those who simply want to know more about Samuel Johnson.