Green’s Dictionary of Slang

(US Und.) a person who provides protection, as well as call services, for madams; thus n. booking, procuring.

1939		H. Powell Ninety Times Guilty 194: Pete Harris was not a very big booker, but he dealt with the quality trade and made a pretty good living [Simes:DLSS].
1939		K.R. Cooper Designs in Scarlet 343: That there are even specialists in the pimping and booking trade.
1958		(con. 1930s) Murtagh & Harris Cast the First Stone 198: New entrepreneurs, calling themslves bookers, grew out of the vaudeville-circuit pattern. In addition to running a call service for madams, they organized a protection racket that was tied in with the bail bond business.