We dealt with this in a previous chapter.
Am I right supposing that the indefinite article used with the word "previous" means "in one of the previous chapters, not necessarily in the chapter that is directly before the one we are going to read today" (otherwise the article is definite)?
Yes, you're right. And there's something subtle going on here, too, which will shed light on a variety of other English constructions.
If the author said "the previous chapter", that would mean the chapter directly before whatever is understood as the current chapter. If the current chapter is chapter 9, then the previous chapter is chapter 8.
Since the author did not say "the previous chapter", that means that the author probably does not mean chapter 8. Saying "a previous chapter" suggests that the author means any of chapters 1 through 7.
You might occasionally find that violated for this reason: While writing a book, sometimes the author writes a later chapter before writing an earlier chapter. While writing chapter 9, the author might be planning to cover a certain topic in a previous chapter, but doesn't know which chapter until actually writing the rest of the book. In that case, the author writes "a previous chapter" to avoid saying a specific chapter. So, in this circumstance, it could be chapter 8. A good copyeditor will replace "a previous chapter" with the specific chapter number, but that kind of polish isn't feasible for all books.
Still, the main lesson to learn from this is that when one article has a very specific meaning in a given context, choosing the other article often suggests excluding that very specific meaning. Similarly for other sorts of grammatical choices in English. What the specific meaning could be varies greatly from context to context, so there's no hope of making a rule for this, but it's part of the way people use articles to communicate some subtle things very concisely.
Here's another example of the same thing. Suppose that you work for a newspaper. You're at home and the phone rings. Your roommate answers and hands you the phone, saying "The newspaper is calling." That means the newspaper you work for. If your roommate says "A newspaper is calling," that means some other newspaper, such as a competitor who wants to hire you away.
"A" here stands instead of "some", however, it doesn't imply the exact same meaning that "some" would have.
We dealt with this in a previous chapter. - means that there was some chapter before the one we're doing now that had the information we dealt with.
We dealt with this in some previous chapter. - is likely to point to that a certain chapter before the one we are doing now wasn't of very much importance, probably, or isn't known any more; we can't surely know which one it was.
