The Case of the Missing Message(1959)
(The first book in the Brains Benton series)
A novel by Charles Spain Verral
The first in a series of six titles features Brains Benton and Jimmy Carson as the Benton and Carson International Detective Agency. Already pre-established as good friends and partners, with a number of small cases under their belts, this is their first "big case"... and it is a pretty big case too, involving a circus and a dangerous man by the name of Otto, and his equally nasty colleague known as the Joker.
Used availability for Charles Spain Verral's The Case of the Missing Message