Correct Polarization of Common Source Amplifiers
The author of the book tries to reveal the "philosophy" behind the transistor differential amplifier (aka long-tailed pair) by building the circuit... and not by giving the ultimate perfect circuit solution... which deserves our respect. Here is what this "building scenario" would look like briefly:


We can drive a (FET) transistor from the side of the gate (common-source amplifier), from the side of the source (common-gate amplifier)... or from both (differential amplifier). So the bare transistor is itself a differential amplifier with two inputs - inverting (the gate) and non-inverting (the source). Then let's apply the two (grounded) input voltages VA and VB respectively to its gate and source.
The problem of this "1-transistor differential amplifier" is that the input sources do not work under the same load conditions - VA is practically unloaded (it "sees" the extremely high input impedance of the gate) while VB is significantly loaded (it "sees" the very low output impedance of the source)... ie, the trouble is in the non-inverting input (the source). Then let's help it.
If we connect a(nother) source follower (acting as a buffer) between the VB input voltage source and the source, we will "alleviate the suffering" of VB and put it under the same conditions as VA. Thus both they will "see" the same input impedance.