Alvin's Secret Code (en Inglés) - Clifford B. Hicks;Bill Sokol

The warm, tingly feeling that had been smack in the middle of Alvin Fernald's stomach ever since Miss Peppersmith had closed and neatly lined up her books that Friday afternoon, had grown bigger as he and Shoie walked home from school. 

Now, here, in his hand, is the reason why! Alvin's sixth sense tells him that the crumpled note holds a secret message in spy code, which is about to catapult the two 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 end.

Book 2 of 4 in the Alvin Fernald series.