For example, commercial ebooks and electronic version of Amazon books?
Does the copyright owners fire complaints to those services? My assumption is that they DO but does the hosting service providers have to remove the resource? Is it against the law if they don't? They must have some kind of disclaimer, but does this really disclaim them of the possible infringement of hosting these copyrighted materials?
I know a lot of forums where members share with each other copyrighted ebooks, in this case, will the forum owner be caught in law?
Yea the DMCA is law for that but as you mentioned many forums or illegal sites focusing on "warez" have been adding more and more online books to their download pages.
It's getting ridiculous in my opinion but whatever...
I would not recommend this. You can be issued a cease and desist notice and get creamed in court. You know what you're doing when you're doing this, and you can be held liable if the site is tied to you.
Granted, there are people who do this. Usually, they host sites in countries where the law is different. They also live in those countries. In other words, they're avoiding the law, for the most part.