Can you elaborate a bit on how to use the JFET as a multiplier?

Yes, that went by pretty fast.

Here is how to make a multiplier with a JFET (and an op-amp): make an amplifier op-amp circuit, such that the gain is proportional to the feedback resistor $R$. If this feedback resistor is then a variable resistor with value determined by the second input voltage, then the output will be proportional to the product of the two inputs.

A JFET in the linear regime (see Fig. 5.2 of Eggleston) is effectively a variable resistor with value determined by $V_{gs}$ (i.e., the slope of the $I_d$ vs $V_{ds}$ curve depends on $V_{gs}$). So this is what you would use as your feedback ``resistor''.

In practice, one can get integrated circuits that do multiplication (and that are made directly out of transistors on a chip).