I am not convinced that the current iteration of progressivism is confined to a minority of "overly-online often out-of-touch people". Progressive leftism perhaps, but overt "leftists" are a relatively niche minority even online. The difference is that their belief system isn't openly bigoted, so they don't feel the need to hide in the shadows as much as, say, the alt-right. But that's also evidence against the both-sides equivalence you are proposing in the first place.

When it comes to published literature, you might be disappointed to find that there is a lot of radical leftist literature out there, just as there is a lot of radical right-libertarian, traditionally conservative, religious fundamentalist, classical liberal, etc. literature out there.