Of the many books written about Agatha Christie, the following have been most helpful:
Barnard, Robert, A Talent to Deceive (1980)
Campbell, Mark, The Pocket Essentials Guide to Agatha Christie (2006)
Morgan, Janet, Agatha Christie (1984)
Osborne, Charles, The Life and Crimes of Agatha Christie (1982)
Sanders, Dennis and Lovallo, Len, The Agatha Christie Companion (1984)
Sova, Dawn B., Agatha Christie A to Z (1996)
Thompson, Laura, Agatha Christie, An English Mystery (2007)
Toye, Randall, The Agatha Christie Who’s Who (1980)