For example, a sentence I just read went as follows:
> “‘I don’t think [the book] had anything to do with his arrest and neither does Anne Marie Schubert,’ he says.”—(Excerpt from the appendix of ‘I’ll Be Gone in the Dark’ by Michelle McNamara)
Found in Other.
the words in the brackets have been added or altered by the author not the person/ have not been said by the person quoted
They’re changing part of a quote from something understood in context of the quote to something explicit. In your quote [the book] was probably “it” or similar.
It’s usually done to show words that have been inserted, usually for clarification. These can be places where a word was accidentally omitted, or a word like “it” was used and they’re replacing “it” with what the author was referring to, while still showing it wasn’t what was originally written. It’s done just to help things make more sense.
Brackets are used around words when the author is directly quoting but those words inside the brackets don’t appear in the quote. In the example you cite, instead of The words “the book” the direct quote may have contained something like the actual book title or some other way the person being quoted was referring to the book. In this context, brackets are a standard literary device used to indicate that the author is inserting words in a direct quote, but that insertion is not supposed to change the meaning in any way.
Those brackets indicate an inserted section, usually where when a word like “it” is used to, to explain what”it” was referring to. This is done for clarification purposes.