Causative form and willingness of the cast

Question

According to "A dictionary of basic Japanese grammar" pg. 389 (on the choice between に and を particles with causative form)

When ni is used, the causee has taken an action intentionally.

I have trouble understanding that, as they give an example of

父は私を(not に) むりやりパーティーへ行かせた。

Supposedly に is ungrammatical in this situation as the subject did not intend to go to the party. But aren't the vast majority of cases where you want to make someone do sth. involuntary? At the same time, in the Genki II textbook (second ed. pg. 234-236) there are examples of sentences using "に" where it is implied that a person made someone do sth. against their intention, like

お母さんは子供に本を読ませました

So my question is what are the actual rules of using に with causative verbs.

Answer 1

Your book quote is specifically for intransitve verbs. Page 390 of the same book says:

If the main verb is transitive, the causee can only be marked by ni.

In your example お母さんは子供に本を読ませました the verb 読む is transitive.