Active Reading Solution: A Logic Gate

In order for current to flow from the "High V" lead to the "Low V" lead, it must flow through both transistors: first the top one, and then the bottom one. The book chose the convention that a high input voltage to a transistor allows current to flow, while a low voltage blocks current. (Remember that a transistor could easily be made the other way around.) Based on that convention...

If both transistors have high voltage, current will flow through both; you can think of the entire two-transistor assembly (above the resistor) as just a wire. So the output lead is at the "High V" voltage. In this scenario, the voltage drops from High V to Low V across the resistor.
But if either (or both) transistor has a low voltage, it blocks the current. Because no current flows through the resistor, both sides of the resistor are at the same potential as each other, so the output lead is at the "Low V" voltage. In this scenario, the voltage drops from High V to Low V across the transistors.

This is therefore a design for an "AND gate"; the output lead is high if, and only if, both input leads are high.