The Heterodyne Boys and the Pneumatic Oyster
is one of the published adventures
of Bill and Barry Heterodyne
which either didn't happen, did happen and has been highly distorted, or actually did mostly happen.
The cover has Philbert Olio as the author.
Agatha picked it up in Gil's library and read "Hey, Klaus, what are you doing in the tar vat?" "You put it under the hatch, you great idiot!" Punch scratched his head. "Wull, iffen you hadn't been runnin' away... ✣ " before realizing why Gil might be as reluctant to share the stories showing his father in a poor light as Agatha is with the stories mocking Punch and Judy.
(The tar vat falls under "highly distorted" at the very least.)