What purpose does any capacitor have? To pass AC and not let DC through. SO when it's used as a coupling capacitor, it lets the AC signal pass between stages, but blocks the DC from the first stage from getting into the second stage where it would upset bias. When used as a bypass capacitor, it in effect shorts the AC signals to ground, without shorting out the DC voltage.
Of course, a capacitor's ability to pass AC varies with frequency and capacitance. A small value capacitor won't do well at passing low frequencies, while a high value capacitor will.
Now given that information, that should have been covered in the books, why do you think the capacitor is there? Think about why it might be different from having the resistor going directly to ground.
Michael