You should understand that what a voltmeter measures, in principle, and the electrostatic potential we are talking about in these sections of the book are the same physical quantity. But you cannot use a voltmeter to measure the electrostatic potential due to a collection of charges that are just sitting around stationary in the lab since that is not the way that voltmeters really work. Normally, you attach the two probes of a voltmeter to different places on a circuit. The voltmeter contains a really large resistor and measures the current flowing across that resistor. The scale on the voltmeter automatically converts the current reading to a voltage reading using $V=IR$. If instead, you try to measure the the potential difference between two points in air (or, even worse, vacuum), the resistance of the air between two voltmeter probes is even larger than the resistance in the voltmeter, exceeding its design parameters.