What's the Name of That Book??? discussion

Alvin's Secret Code by Clifford B. Hicks? (hardcover book published in 1963)
Alvin's sixth sense tells him that the crumpled note holds a secret message in spy code, which is about to catapult the two seventh grade secret agents plus Alvin's younger sister Daphne, the Pest, into the intricacies of authentic code detection and the dangers of a newly stirred-up 100-year-old mystery.
Even Alvin's Magnificent Brain is nearly put on overload before the bang-up solution to the code and the mystery carry the three agents to an ending that is satisfying to all...well, to almost all. This classic tale from 1963 includes Alvin's own notes on cryptography in an appendix at the back.

Their neighbor, Mr. Link, helps them decode the message.

I Googled "children's book" "codes" "ciphers" "1970s" and the first result was another reader asking for a book that sounded like yours:
http://w1.loganberrybooks.com/stumpth...
Several people answered "Alvin's Secret Code" so you can thank them :-)