A couple from my childhood:

- _Codes & Secret Writing_ by Herbert S. Zim

- _Codes and Ciphers: Secret Writing Through the Ages_ by John Laffin

- _Codes, ciphers, and secret writing_ by Martin Gardner

There was another book on alphabets/letterforms and ciphers --- pretty sure it had a cover designed by Oscar Ogg, but blanking on the name and can't find it.

While searching online for that found:

Secret Writing: An Introduction to Cryptograms, Ciphers, and Codes by Henry Lysing and John L. Nanovic

Cryptography adjacent (it's about the difficulty of deciphering a dead language), but H. Beam Piper's novella "Omnilingual" really should be part of the grade school canon --- lightly updated version at:

https://vrici.lojban.org/~cowan/omnilingual.html