I can't give any links, but I know how regulated supplies are designed. A transformer decreases an AC input from line voltage to about 12 to 20 volts AC. This is then rectified using 4 diodes arranged in what is called a wheatstone bridge. This changes the AC sine wave into a series of DC pulses. These pulses are smoothed with a capaciator which runs from the positive rail to ground. The result is a steady DC current with a minimum of ripple if the capaciator was the right size. However, the current is still unregulated at this point. A special transistor called a voltage regulator is placed between the positive and negative rails. It works to resist changes in the voltage if the resistance of the load connected to the output changes. Any excess current is turned into heat and dissipated using a heat sink. More efficient regulated power supplies use Phase Width Modulation (PWM) to regulate voltage. PWM can also be used to control the speed of AC motors, which can't be done by simply decreasing the input current. The design of power supplies is one of the first things learned when actual circuits begin to be designed. Any EE textbook is sure to have several chapters devoted to power supply design.
Supplies with more than 1 voltage setting accomplish this using a center tapped transformer. If the output is 12 volts, then the rectified center tap can give zero volts (ground) and the other taps will be either + or - 6 volts. Remember, voltage is a difference and represents a pair of values, not a single one.