Poe writes, "It is only with the denouement constantly in view that we can give a plot its indispensable air of consequence, or causation, by making the incidents, and especially the tone at all points tend to the development of the intention." He writes toward one goal. Before beginning a work Poe asked himself which effect "of which the heart, the intellect, or (more generally) the soul is susceptible" he wants to impart with the work. Then he considers how best to invoke that effect in tone and incident. Poe believes that reading in one sitting is important for singularity of effect, otherwise the real world happenings between readings will ruin the unity.
Although Poe's ideas are directed specifically at poetry and shorter works appropriate for one-sitting reading, he makes it easy to apply the idea to longer works when he writes, "What we term a long poem is, in fact, merely a succession of brief ones--that is to say, of brief poetical effects." We don't expect readers to devour a novel in one sitting. Even if we've done it, it is unreasonable to expect everyone to have that kind of free time. When considering the effect of your novel, think about the effects leading to that overall response. What feelings do you first need to take readers through in order to leave them with that final effect? Each of those feelings relates to a scene and each scene can be treated as Poe treats the "succession of brief ones" that make up a long poem.
The unity of effect concept is particularly useful when you're editing. In a first draft, there is typically far more material than necessary. To trim the fat from your novel, ask a series of simple questions:
-What effect am I hoping to achieve in this sentence/scene/chapter/novel?
-How does it support or lead to the overall effect?
-Do these words, as they are, create that effect?
-Do they have any effect?
Editing this way makes it easier to find the extraneous words, phrases, or scenes in your novel. Try this the next time you're not sure if a scene should stay or go.