Ok, I've been reading the book some more and I think it's starting to click.
Let me see if I have this right so far:
The oscillators in the tv are normally scanning the screen by themselves, but the sync pulse from the video signal temporarily "overrides" them.
The composite video signal uses a negative voltage, and the amplitude determines the brightness. Lower amplitude means a brighter image, and a higher amplitude means a darker image. The amplitude varies the voltage on the control grid on the CRT, which is why a higher voltage reduces the intensity of the electron beam and darkens the picture. A higher voltage on the control grid repels electrons from the cathode. The picture signal by itself (or what the book is calling the "camera signal") is continuously varying the voltage on the control grid to create the light and dark parts of the picture.
The blanking pulse comes at the end of a scanned line, right before the sync pulse. It's a high amplitude, so it reduces the intensity of the beam all way down to make the horizontal line blacked out so that retrace lines don't overlap a line that was just scanned. Then the sync pulse makes the electron beam get deflected or "retraced" back to the left. Then the next line is scanned. I guess it's timed perfectly so that right when the beam is retraced back to the left, then the vertical deflection is about to move the electron beam down to scan another line.
Am I understanding it right?