This book is a lot of fun, and may even be of some use. A number of ciphering techniques are outlined, and the interested hobbyist can undoubtedly come up with many variations on the themes. Of the five chapters that actually deal with encipherment, three deal strictly with mono-alphabetic substitution. The remaining two chapters deal with poly-alphabetic substitutions that are still, because of the fact of substitution, subject to brute force attacks.
Advanced encryption is currently the province of higher mathematics.
Explanations and sample code for these would require more sophistication than the current book demands. Still, it would not have been impossible to include them, and it might have improved the scope of the book. Simple, and subject to attack, or not, the techniques in the book can be used for some measure of privacy and security. As stated in the preface, even crackable codes may raise the expense of getting at the data beyond its worth.