From OED for the adjectival sense of "booked":
4.a. Of a seat, room, appointment, etc.: engaged or arranged in advance; reserved.
So I think it means the passenger who already has a reservation on the coach. "by this history" refers to the preceding recounting of the story; one of the passengers has a booking, the other two don't.
I didn't see anything in the earlier text that actually describes a passenger having a booking, so perhaps it's meant figuratively to refer to the passenger who was singled out in the preceding text.