Childrens Mystery book. Story with codes and ciphers in the back, including a wheel and most commonly used letters. At least one boy and one girl. Read in 1990s, probably written 1-15 years before that.

[Ruth]

I think there was at least one boy and one girl solving the code

[Ruth]

Mystery book about codes and ciphers

[Kris]

Ruth, can you tell us more about the mystery and how the kids get involved? Location?

Perhaps like a variation of a Caesar cipher wheel for replacing letters? Do you recall the other types of codes?

I added a few details to the header/ topic title. Feel free to edit it.

[Simi]

Might it be one of the books in the Can you solve the mystery series
Just be aware that the covers on goodreads are of the modern editions. This series has been out since 1983 and underwent various changes such as the name. It's been called Hawkeye Collins & Amy Adams as well as the Piccolo MysterySolvers in the past.