Current Direction in Mosfet
The reason reference books say that N-channel current flows from drain to source is that when the transistor is OFF the intrinsic diode of the transistor pevent the current from flowing from source to drain. When the transistor is ON, the current can flow in either direction as the diode is effectively shorted by the Rds(ON) of the drain to source channel.

Similar effect takes place in a P-Channel transistor.

In the shown circuit the N-Channel FET diode prevents current to flow from the output back to the input in a paralleled supplies configuration. The FET get's turned OFF when the input voltage drops below the output voltage minues some small voltage. In normal operation the FET is ON, bypassing the internal diode and ensuring minimal losses.