Alvin's Secret Code by Clifford B. Hicks. That's the one that got me started. I think I read it in 3rd grade (age 8).

Thanks for the recommendation, always great to hear of books begin the journey.

Came here to mention this. It's not too applicable to modern/useful cryptography, but introduces a bunch of related hidden-information concepts like product date codes, which I think I actually use far more often.