The effect is mysterious (if not acknowledged)

Point 4

There is a mixture of oratory or propaganda in every practical book...Since your ultimate judgment of his work is going to turn on your acceptance of the goal for which he is proposing means, it is up to him to win you to his ends.

As Adler points out, there is absolutely nothing wrong with this so long as the author uses legitimate means for persuasion.   Dialectic and Rhetoric are tools for reaching probable knowledge -- that is, in areas where absolute certainty is unobtainable.   Most matters of judging "better and worse" -- that is, most areas in which Practical Books are concerned -- cannot be decided with scientific certainty.  

This is from the Wikipedia article on Aristotle's Rhetoric. 

Indeed, the first line of the Rhetoric is"Rhetoric is the counterpoint of Dialectic."

Logic, to Aristotle, is the branch of philosophy concerned with reasoning to reach scientific certainty while dialectic and rhetoric are concerned with probability and thus are the branches of philosophy best suited to human affairs.
...
Dialectic and rhetoric together create a partnership for a system of persuasion based on knowledge instead of manipulation of emotion.

Now, Adler's view of persuasion in this chapter seems to be subtly different from Aristotle's.   He writes:

"It is of the very nature of practical affairs that men have to be persuaded to think and act in a certain way.... No one makes serious practical judgments or engages in action without being moved somehow from below the neck.  The world might be a better place if we did, but it would certainly be a different world."

Where I seem to see a subtle difference is in that Aquinas, at least, would think that because of how we are made, our appetites lead us towards the good.   Our appetites are not all physical so there is no "above the neck/below the neck" dichotomy.   Scripture speaks often of the "heart" which seems to me to imply a meeting point of all our faculties, spiritual and physical and intellectual.  Aristotle, it seems, would think that persuasion is meant to increase knowledge, not just manipulate the emotions. 

Adler goes on to say that
(1) the practical writer who does not acknowledge this will be ineffective.
(2) that the reader who does not acknowledge this element in practical books will not be equipped to deal with it.

"Propaganda taken in that way is like a drug you do not know you are swallowing.  The effect is mysterious; you do not know afterwards why you feel or think the way you do."

Whether the emotional system is to be deprecated, acknowledged as an innate part of our rational human nature, or celebrated as the way in which the Creator chose to make us, it certainly is an empirical fact and this is where Adler overlaps with Aristotle and Aquinas.  

The three traditional modes of persuasion are logos, pathos and ethos.   Aristotle says:

Persuasion is clearly a sort of demonstration, since we are most fully persuaded when we consider a thing to have been demonstrated.

Of the modes of persuasion furnished by the spoken word there are three kinds. ... Persuasion is achieved by the speaker's personal character when the speech is so spoken as to make us think him credible.... Secondly, persuasion may come through the hearers, when the speech stirs their emotions. ... Thirdly, persuasion is effected through the speech itself when we have proved a truth or an apparent truth by means of the persuasive arguments suitable to the case in question.

Aquinas says that our emotions help or hinder our efforts to do better or worse.  In other words, love for something speeds us towards that thing, fear contracts us so we tend to shrink from the thing, hatred or aversion repels us from that thing, hope inclines us to work towards a thing that is loved and desired.   So perhaps  rhetoric or persuasion is perfectly legitimate when it moves your emotions RIGHTLY, but it is distorted when it strives to move you by fear, or love for an inferior thing, or hatred for something that is not really hate-worthy.   Basically, it is distorting when it strives to move you in a direction that is not reasoned; but emotion in the service of reason is a fine thing and also the way God intended us to operate, since He didn't create us as pure spirits, like the angels.

You see that in exemplary form in Jesus's life and way of teaching -- He had no problem operating with the full range of emotion and teaching in a way that included emotional power, but He never mis-used emotion in His life or teachings.

So another set of questions to ask in regard to a practical book.

• What means does it use to persuade you to its ends?   
• In what way does it use reasoning or demonstration of truth? (logos)
• In what way does it strive to stir the emotions of the reader? (pathos)
• In what way does it employ the credibility of the writer? (ethos)

Narrative and anecdote are often used in a practical book.   In fact, it's very difficult to read a practical book that doesn't have at least a few concrete examples.   Narrative is the core of rhetoric in many respects and also has an intriguing therapeutic aspect.    It is said to unite logos, pathos, and ethos in many ways -- as you can see very well in Jesus' parables.   So perhaps I would add another question:

• In what way does a given practical book employ stories, anecdotes and examples to illustrate or demonstrate its points?