Internal Dialogue by Marcy Kennedy is a good primer on its title subject: Internal dialogue. This is not just about how to show it. I suspect most writers understand this. It also shows the basic techniques of when and how to use it.
An added bonus was a section titled “Alternating the Focus of Paragraphs.” This gave invaluable advice on a certain pacing or balancing of the paragraphs. I won’t repeat it here; you’ll have to get Kennedy’s book, as a kind of reward to her for this.
I understood the pacing of words within a sentence, and the pacing of sentences within a paragraph. I also understood the pacing of chapters and overall story structure. Internal Dialogue supplied an essential piece that was missing from the middle.