The comma belongs inside the quotation mark. The writers of those books are just lazy and want to see the rule changed to fit their mistaken sense of correctness.
Punctuation is not part of the quote, it is only for sentence structure. For example, if Sally told you there was fighting in the area she would't say the word "comma" out loud, but a comma would certainly be enclosed in the quotation depending on how you wrote the sentence.
"There was fighting in the area," Sally said.
Sally said, "There was fighting in the area."
Both versions are correct, even though they have different punctuation within the quotation marks. The fact that the quote is being taken out of context is no different. If she really said, "There was fighting in the area last night," but we are electing to exclude the "last night" portion of the quote, the punctuation choices are the same as shown above, even though there is no punctuation between "area" and "last" in the original quote.
Further, it does not matter if the quote is comming from another written source. However, if you are going to quote something at length, you might consider skipping the quotation marks altogether and indent the quote instead.
There was much fighting in the area that night, but it was not more
than any other night in those parts. We were getting used to the
violence and now that you mention it, it would be rather lonely if we
didn't have all the screaming and hollering going on out there.
This makes it absolutely clear to your reader that everything in the indent is exactly as the original author intended it. Some books will even change the font type, but indenting is all you need.