TwiLyghtSansSparkles

Yes, why?

I could understand humming incessantly if you worked in, say, a sea of cubicles, but at a library? 

That's just not the sort of thing you do at a library. He's just creepy, is what he is. 

Next time you catch him doing something, you could try politely confronting him. 

Like, "Hey, [Spongebob! coworker], I've noticed you humming a lot recently. Could you maybe keep it down, or stop doing it, because it's a bit bothersome. Thanks." 

I've confronted him before, on his looking over my shoulder, and he denied doing it to my face....shortly after doing it. If I confronted him on the humming, or the proselytizing, or the political talk, I'd probably get an attempted guilt trip, too.