Linking to printed books on archive.org I am a postgrad PhD student. I occasionally participate in answering academic questions a Q &amp; A site when I hit a block with getting on with my thesis. 

I often cite a particular large and well-known academic source in my answers. I have my own copy of this book. It costs between £180 and £240 and is therefore beyond the reach of anyone who isn't a very serious enthusiast or a serious academic. It is a very important resource in my field of study.

Recently, I found that this resource is available online in pdf form at a not-for-profit website called Archive dot org. I have done some cursory research into that organisation. It has an entry on Wikipedia. I can't find any substantial criticism of it.

I am wondering whether it would be academically unethical of me to link directly to this resource in my answers, given the unknown copyright status of some of the content on archive.org. On a related note, I'm also worried whether it could get me into trouble academically to do so. Any insights would be appreciated!
I occasionally participate in answering academic questions in online forums when I hit a block with getting on with my thesis.


No worries, we all procrastinate (ehm... cough cough).


  I often cite a particular large and well-known academic source in my answers. I have my own copy of this book.


You really don't need to own a copy of a book to cite it: many, if not most, academics borrow the books needed for their work from the libraries. Moreover, its cost is irrelevant to your question.


  I am wondering whether it would be academically unethical of me to link directly to this resource in my answers, given the questionable copyright status of archive.org.


If the archived copy is legal (e.g., the publisher might have released the copyright because they no longer print it), it is certainly ethical to link it. However, I'd avoid referring to the copy if it's legality cannot be reasonably established. In addition, should the copy be removed due to copyright infringement, you would end up with a series of broken links.

It is worth noting that links to books are seldom seen on academic papers, unless a book is exclusively available online. Therefore, the issue is more related to whether or not posting the link online.