Mysterious Code, The (Trixie Belden S.)

Principal Stratton wants to get rid of all clubs in Sleepyside High School, including the Bob-Whites! The only way they can save their club is to prove that they help out other people, too. Trixie quickly comes up with a plan to host a benefit antique show and sale. But are the precious antiques in danger from thieves?
From the Hardcover edition.

In the 1940s, Julie Campbell was running her own literary agency when Western Publishing put out a call for talented authors to write mystery series for kids. Julie proposed the Trixie Belden series and wrote the first 6 titles herself, but books 7–39 were written by a variety of writers all under the pseudonym Kathryn Kenny.

Michael Koelsch works in many different styles for many different markets, including advertising, entertainment, and publishing. He recently received a Silver Medal from the Society of Illustrators, NY.
From the Hardcover edition.