This was an admonishment to William's father Robert who had taken a crowded train home from Bradford one morning, leaving his overnight bag on his seat whilst briefly repairing to the buffet. On getting back to his compartment he discovered the bag had been uploaded to the luggage rack, and a dour-looking native had appropriated his seat ...
For best effect, "bum" and "book" should be said in proper Yorkshire, as in the BBC sitcom saga Last of the Summer Wine – particularly as spoken by the combative Nora Batty.