I am currently editing the string-theory tag wiki, and I am adding some links to freely downloadable, copyrighted books at some illegal pirate resources located in third parties.
However, is this considered a violation of copyright policy of stack exchange? Is it allowed?
I won't address the legal issues (because I'm not qualified), but in general, it's not a violation of any SE policy to link to anything.
However, in the interest of maintaining professionalism, I would suggest that if you want to identify a specific book, you should link to the Amazon product page or some similar canonical identifier, not to a site that offers the book's content for download. The exception is if the file is made available at no cost (and with no login requirements or other access restrictions) by the copyright holder (author or publisher), then you can link to the page where this authorized copy is available for download.
As a courtesy to users of the site, you should link not to a PDF file itself, but to a "landing page" which includes a link to the actual PDF file. (Or whatever the actual file is, if it's not a PDF.) This is analogous to how, when linking to papers on arXiv, you always link to the abstract page, not the PDF.
"Open-source" isn't the same thing as "doesn't cost money." For example, if my friend lets me smoke some of his marijuana for free, that doesn't make it open-source marijuana. Open source, in the software world, means that you can look at the source code of the program (not just the compiled version), and the source code is also either public domain or available under a license approved by the Open Source Initiative, opensource.org. When applied to books, the term would imply something similar, except that instead of source code we mean something like a format that is designed to be edited (e.g., LaTeX) and whose specs are sufficiently open that there are open-source software implementations.
By these definitions, sites like gen.lib.rus.ec have nothing to do with open source. Here is a site I run that catalogs books that actually are legally free and/or open-source: http://theassayer.org
is this considered a violation of copyright/bla-bla-bla policy?
In most countries, linking is not a violation of copyright, even if the site you're linking to violates copyright. And of course if the site you're linking to really is a source that carries open-source books, then there is nothing illegal going on whatsoever.
Is it allowed?
All I would suggest is to be honest about what's going on, and not to call it open source if it's not.
