An alternative is plain old school irc. I'm not kidding.
Connect to irc.irchighway.net then /j #ebooks then !search <author and/or title>
Receive search results via xdcc which gives you the commands to download that stuff.
Or query one of the bots (e. g. @bsk) and get a list of hundreds of thousands of books that are available and search that for what you want.
I prefer that over ad infested one click hosters with countdowns, link shortener cascades and captchas. I find and get the stuff much faster by using irc.
#ebooks is composed of datahoarders that have a lot of stuff available. You declare the data source you're getting the book from (e.g. Oatmeal) and then the name of the book.
I'm not sure if #ebooks mirrors Libgen. I wouldn't be surprised if they copy from each other.
I have allergies against ads, countdowns, etc. The interweb is polluted with that stuff and so are most piracy sites.
I find irc to be much easier to use than the alternatives. Just a click to start hexchat and then it auto-connects to the right server and auto-joins the right channel. All within seconds. I even get my animes from irc like I did in 2005. 😁
It's been offline for like a month, maybe more, I haven't kept track. I don't know any details, maybe there's info on their forums, but overall the owners are quite secretive.
For years they've managed to keep their indexing (think html, not torrent) online by separating out the indexing from the actual storage. They've only changed URLs once in recent memory. Throughout the years they've stored content in various places. Protected safe havens, compromised websites, and ipfs.
There are still a lot of places in the world where hosting a link to data is not illegal.
Well through whatever methods required it looks like the publishers finally managed to shut down the search/hosting on wherever .is was pointing to.
There is a static IPFS index laying around somewhere so technically the content's still quite available. Along with Torrents.
Well they finally managed to bring it back up somewhere and I don't even care to go and figure out where that is. Just glad they're still around.