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Source: https://writers.stackexchange.com/a/42683
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

There are two issues here: plagiarism and copyright.

Plagiarism is when you copy from another writer (or composer or whatever, but we're talking about writing here) without giving proper credit. It is very easy to avoid plagiarism: Include a proper footnote.

Copyright violation is when you use someone else's work in a way that could deprive the legitimate owner of sales of his book. If you copied someone else's book word for word and tried to sell it without getting permission, that is copyright violation. If you put a footnote on the last page clearly stating that this entire book is copied from someone else's book, it wouldn't be plagiarism, but it would still be copyright violation.

There's a clause in copyright law called "fair use", that says you can copy short portions of other people's work in certain limited circumstances, which include, for a book review, for educational purposes, or, as relevant here, for scholarly purposes. i.e. you can quote excerpts from someone else's book as evidence or examples of some point you are trying to make or event you are trying to describe, in order to debate opinions expressed, etc.

There is no fixed rule about how much you can copy. If someone sues you, the courts decide on a case by case basis. If half your book is made up of lengthy quotes from one other book, you would almost surely lose an infringement suit. But if you copy a few sentences here and there, from a variety of sources, and you surround them with enough of your own words so that the final product is clearly yours with some quotes from other people, and not just a rip-off of someone else's book, you should be fine. Odds are no one would challenge it, and if they did, you would almost surely win.

I'd say to err on the side of safety. Quoting a sentence or a paragraph here and there is fine. Copying several pages of text is not. Copying an entire chapter would almost surely be over the line.

Original score: 2