ESCs are used for brushless DC motors (BLDC). But PWM is used in both DC and BLDC. The topology of a ESC consists of a 3-phase inverter with back emf feedback to know when to switch (complicated circuitry).
Commercial ESCs are designed with a PWM signal input. This happens to be the same as per the servo motors. They take a 50Hz PWM that ranges from 5% to 10% Duty Cycle. 5% being the motor is off and 10% the motor is going at max rpm.
Brushed DC motors have a minimum operating voltage, so the minimum duty cycle must be chosen carefully as it may lead you to waste power when the motor is not moving. Usually brushed DC motors have 2 signal wires and an enable pin. The PWM would be connected to the enable pin. Every IC is different so always look at the datasheet of your chosen motor driver.
I can't help with book recommendations but you may want to have a look in a library for "electrical machines" with "drivers" and "fundamentals" or "principles" as your main keywords.
Summarizing: PWM is used in both BLDC and DC motors for speed control.
Hope this answer your question about PWM.