I disagree with Al Collins' assertion that plagiarism implies intent.  To continue the simile, plagiarism is like manslaughter rather than like murder.  It's what you actually do, not limited by what you intended to do. 

All university-level scholars know that plagiarism is one of the cardinal sins of academic life.  A significant part of our time as teachers is spent drumming it into our students' heads that they can copy stuff, but it *must* be clearly flagged and attributed, and there are limits.  Plagiarism is one of those academic wrongs like moral turpitude, for which people lose their jobs.   As the WikiPedia page says,
Precisely because plagiarism is a very big deal, and precisely because we all know about cryptomnesia and false memory (at least, that we're fallible in general terms), academic writers and academic editors must take responsibility for checking written work for plagiarism, just as they check for spelling errors or data errors.  You couldn't defend the publication of a paper full of mathematical errors on the grounds that you never meant to make those errors, and they were unconscious! 

I fail completely to comprehend an argument based on the idea that a book of revisionist historical and intellectual advocacy may be allowed to contain plagarized passages because it does not qualify as research.  Plagiarism is a mechanical process of copying without attribution: it doesn't matter whether it's in a book or in a bus ticket.

It's your responsibility as an aspiring participant in human intellectual life to make your work as good as possible, and that includes not copying out other people's work and passing it off as your own.  Consciously or unconsciously, for research or for mere advocacy.

Best wishes,
Dominik Wujastyk