How does the "pendulum effect" affect biplanes?
It will affect a biplane just as much as a monoplane.

Not at all.

The pendulum effect does not exist in airplanes. It does in airships, but not in heavier than air craft.

For a proper discussion, we should first define what a pendulum is. Only then can be established if such an effect can exist in airplanes.

Let's base the definition on Wikipedia. It says that


  A pendulum is a weight suspended from a pivot so that it can swing freely.


Maybe it is also worth to look closer what a pivot is: A thing on which something turns.

So the pendulum is fixed to a fulcrum which keeps it suspended and allows it to swing freely. The ideal pendulum has all its mass in its massive bob and, therefore, pivot and center of gravity are not in the same place. If the center of gravity and the pivot would fall together, a pendulum could only rotate but not swing. And that swinging motion is what the pendulum is all about.

Now for airplanes: Here we have no pivot. All rotation can only happen around the center of gravity. This is equivalent to the pendulum with no length which is no pendulum any more.

Lift is the sum of all pressure acting perpendicular to the direction of movement. Lift on a banked wing will also bank with it. The lift vector will still be in the plane of symmetry of the banked aircraft and will have no lever arm with the center of gravity, thus causing no uprighting rolling moment. The Figure 34 of your copied book page is simply wrong. The author did not know what he was talking about.

Edit especially for @JohnK:

I added an answer here about the rolling maneuver of paragliders. The description should make obvious that no pendulum effect is involved. Rather, the whole is quite similar to roll control in hang gliders where the lift is shifted sideways in order to create an imbalance, but then again different in wonderful ways. The discussion below would not allow me to explain my thoughts in such detail.