The style is always interesting. Just so you know. Small interjections capture our attention and focus us exactly when our minds may have been wandering. For each kind of code the procedures for encoding and decoding messages are covered. Even more interesting are Molly's methods for analyzing the codes.
Frequency analysis is covered for letters, digraphs, and trigraphs.
Beth and Stephanie have encoded a message that Molly successfully decodes. Clearly these codes are vulnerable to frequency analysis, leading naturally to a desire for more sophisticated ciphers. The second chapter takes up polyalphabetic substitution ciphers. Factorials, permutations, combinations, and probability are the mathematical foundations for the second chapter. The barest essentials are covered in an interesting way.