The Art of Electronics, figure 2.10B
1)


When Q2 is on, it has very low Vce, 'pulling' the collector of Q2 to near ground.
When Q2 is off, R3 holds Q3 in an "off" state, because it causes the base voltage of Q3 to be almost identical to the emitter of Q3. When Vbe &lt; 0.7V the transistor will be OFF
OFF = no current


2)

Which current? Q2 sinks current from R3 when it's on. The "Load" sinks current from Q3. It's a resistor or a light bulb or something.

3)

When Vbe is less than the cutoff voltage, the transistor will be off.

It's not the most easy to read passage in the book, but you should be able to get through it by tracing currents and voltages on your own, matching up what he's saying with what you see.