This is book number 16 in the The Baby-Sitters Club series.
Jessi knows a secret language! She learned it from Matt Braddock, the BSC's newest charge. Matt's been deaf since birth, and he uses sign language to speak. Since Jessi is Matt's baby-sitter, she's been using sign language, too.
Soon all the kids in Stoneybrook want to learn to sign . . . which keeps the Baby-sitters busy. Jessi's the busiest of all: she working on another secret just for Matt. Will she be able to keep the secret and pull off her special event?