I've actually bought a book on dialog because dialog is very important, and I'm not learning much new on my own. I haven't finished it though, and don't know what they have to say about internal thought, if anything.
I have developed my own rules for italics based on what I've seen and what feels right to me. As a general rule, If I have a very short, one word internal thought, it either leads a line of text, or stands completely alone.
If I mix a single italicized word into a paragraph, it is narrative emphasis.
This might change if the author of the book I am reading is very convincing, and disagrees with me.