1. The Secret of the Old Clock (Book 1): This is the very first book in the series, and it sets the stage for much of the mystery surrounding Nancy's family. Carson is kidnapped by the villainous Mr. A. Fleming to coerce Nancy into giving him a valuable antique clock.
2. The Mystery of the Topaz (Book 13): In this book, Carson is again kidnapped by the villain, a mysterious figure named "The Shadow." This time, the kidnapping is connected to a stolen topaz ring and a complicated web of secrets.
These two books are key to understanding the early Nancy Drew mysteries and the constant threat that her father faces.