Continuity mistake: When the Ghost writer stops the BMW to follow the sat nav there is a car in the background. It vanishes then reappears as the film changes from close up to panoramic shots.
Continuity mistake: When driving to the house for the first time, the taxi stops at the gates for a security check and Ewan is sitting in the back seat on the left (he passes his passport out the left window). In the next shot, as the taxi pulls to a stop at the house, Ewan is sitting on the right with his bag still in the middle of the seat. I doubt he switched sides for the short haul up the driveway.
Answer: Who would want him dead? Ruth, for one, as well as Professor Emmett (her CIA recruiter) and others involved in the conspiracy. Yes, the book had been published, but the secret message only comes across if one is in possession of all the knowledge that the writer had acquired, which the casual reader would not have. And minutes is plenty of time for Ruth, Emmett, or anyone else to make a phone call and arrange the hit.
The published book was a rewrite and would not have the secret message in the chapter beginnings.
The published book was a rewrite of the original manuscript. He gives her the original manuscript that he was working off, not a final draft.
We know the beginning of the book was rewritten as he didn't like it. Almost certainly, the clue is not in the published version of the book. Ruth was shocked to read the note and is delivering her speech so is unlikely to be complicit in the murder of the Ghostwriter. The ending suggests that Ruth, too, is little more than a pawn in the whole conspiracy.
Makes no sense, he was run down 2 minutes after she read it.