Please answer a question about the following article about To Kill a Mockingbird:

Charles Shields, who has written the only book-length biography of Harper Lee to date, offers the reason for the novel's enduring popularity and impact is that "its lessons of human dignity and respect for others remain fundamental and universal". Atticus' lesson to Scout that "you never really understand a person until you consider things from his point of view—until you climb around in his skin and walk around in it" exemplifies his compassion. She ponders the comment when listening to Mayella Ewell's testimony. When Mayella reacts with confusion to Atticus' question if she has any friends, Scout offers that she must be lonelier than Boo Radley. Having walked Boo home after he saves their lives, Scout stands on the Radley porch and considers the events of the previous three years from Boo's perspective. One writer remarks, "... [w]hile the novel concerns tragedy and injustice, heartache and loss, it also carries with it a strong sense [of] courage, compassion, and an awareness of history to be better human beings."

Who wrote the only book length- biography of Harper Lee?
Charles Shields