I've seen dark books at the front of many libraries - I am not talking LGBTQ, I am talking full out spell and witchcraft books. For years they were near the front door of our library, eventually these were moved and now I'm not sure where in the library they hide, but I'd rather not seek them out as the feeling around those books is pure dread and I don't believe should be touched at all. They definitely exist for sure and the first time I saw that about 10 years ago now, I was shocked. The Library Association in America in general is corrupt. They promote anti-God propaganda and they do so quite often if you follow it. There may be some Christian books floating around, but definitely a lot of libraries pushing agenda's they shouldn't. Well-meaning respectable authors have a hard time making it in today's book world.
There is plenty of witchcraft and occult behaviors people indulge in today.
Although the term "666" in general can get complex. It can be a number for "evil" and The Holy Bible mentions that these are numbers involving a name. This has been related to an anti-christ that will make an appearance at some point. It can have different meanings from what it seems. "7" is generally the Biblical number of completion and a repetitive 6 tells us this falls short of God's perfection and perhaps a warning of danger.
There's a lot of debate about the right hand and forehead. Personally, I believe this is literal - for that mark of the beast to be on the right hand or forehead. Again, heavily debated - many interpretations. Do we know who is exactly right? At this point, not really. When it happens, I think we will probably be like, "Oh, wow. Okay. I see now." and know it isn't good.
The Bible tells us "for man's heart is deceitfully wicked, for who can understand it?" It's not saying "the atheist man's heart" or "the unbeliever's heart" - for man's heart, talking about all people. When we become a new person in Christ, we turn away from that as much as possible, but not everyone does. We're all prone to evil if we let it fester in our hearts.