A is for Arsenic: The Poisons of Agatha Christie by Kathryn Markup is a good Christie reference book. Ms. Markup covers the poisons that Christie used when she wrote her mysteries. As you may know if you have read anything about the biography of Christie, she started out during World War I by working in a pharmacy. She learned the chemical reactions of drugs and also learned something about how dangerous some poisons are. A is for Arsenic is not really about the books themselves, but about the poisons. For example, we learn how arsenic acts, and how is it used. We get some information about the mystery (or mysteries) where Christie used the poisons, but the poisons themselves are the stars of this book.
A is for Arsenic is an interesting ready, but I have to admit that I found myself skimming the book. Instead of reading this book as you would a typical fiction book, I think it would be great as a resource for the dedicated Christie fan, who could consult the book as he or she reads the mystery.