some of my colleagues told me that most of plagiarism detection sites cant not spot plagiarism from books or articles that are not available freely online. is that true? if yes, which site can do that ?
From my experience iThenticate has a good reach. It goes not only through research papers and books but internet publications as blog entries and legal documents online. Unfortunately, as any plagiarism detection software it is not informative without human eye input as in case paper is written in a low sophistication level language, the software will show high percentage of plagiarism (mostly common phrases from online texts), when the problem is not plagiarism but simply bad academic writing skills. And the exclusion functions don't work that well, so one needs to be attentive to content of the plagiarism percentage found by iThenticate.