[gojomo]

Reuse can never be 'plagiarism' if you cite your source and present it as a quote. Plagiarism is only presenting someone else's work as if it were your own.

Now, if the book is 99% other people's work, but all copying is cited, you might still run afoul of copyright laws for copying without permission. But it won't be plagiarism. Whether 'fair use' applies depends on a test where many factors are weighed. See:

http://en.wikipedia.org/wiki/Fair_use

Very roughly, the idea is: does your use add value for society without damaging the value of the original work? Using less is better than using more. Using for educational/nonprofit uses is better than trying to make a quick buck. Trying to make a buck in a new transformative way is better than trying to make a buck at the expense of (by replacing) the original work in the marketplace. Using just enough to have a conversation about the work, adding your commentary, is better than just reusing the juicy parts to save yourself effort. Etc.