The publishing industry today is where the music industry was about two or three decades ago.  Libgen is rather like the original Napster.  Libgen makes many books downloadable without any regard for copyright rights or the law generally.  Publishing companies from time to time try to shut down Libgen and its predecessors like Library.Nu.  In the past few months, for example, such a publishers' consortium has succeeded in shutting down many of the mirrors of Libgen (like https://b-ok.cc/).  But it's whack-a-mole.  What publishers are not responding to is the need for a new model of digital book distribution, analogous to streamed music.  We should be able to pay about $10 per month and read whatever we want.  Amazon is the one large company I know that has tried this, with its Kindle Unlimited program.  That way, everyone gets paid, and everyone can read without fear of prosecution.

Where does that leave us?  Downloading books from Libgen is almost always a serious breach of copyright (so always use an anonymizing VPN!).  On the other hand, it is often irresistibly convenient.  And for readers without access to a well-funded library system, it can be the only way to read a book.  If you want to check a single page or quotation, it is hard to justify the purchase of a whole book, especially at academic book prices.   Sometimes, Libgen can allow one to check a reference far more quickly than by accessing the same book DRM-protected and legally accessible through the library system.  And time matters.
There are so many books on Libgen published by companies like Brill and Routledge that it's hard to believe they are not quietly aware of it, and accepting it as an alternative distribution method, although of course they would officially deny it.  So many of the PDFs are clearly publishers' originals, not scans, suggesting at the very least that there is uncontrolled leakage of content directly from the publishers.

We live in interesting times.  Libgen is more than merely "shadowy."  To download from Libgen is in most cases a crime against copyright law (has the author been dead for more than 60 years).  Just as it used to be with Napster and early Spotify.  At the same time, the obscene profits of giant publishing corporations like Pearson, Bertelsmann, Elsevier and Springer and Scholastic, coupled with their unwillingness to adapt to the world of digital distribution, make it almost inevitable that Libgen exists. 

Downloading from Libgen can be handled in a manner that makes detection impossible, and in many cases the breach of copyright is so slight, for example if less than 10% of the book is displayed on the computer screen, that if it came to court it might be judged "fair use."

Proceed with caution, and hope for better business models from future publishers.

Best wishes,
Dominik