I never wear any footwear at all, and it's no problem to be barefoot here in the Netherlands, in the library or anywhere else. Contrary to what many people in the US believe, it not dangerous, unhealthy or gross to go barefoot. Feet get very tough very quickly, our skin is made to keep pathogens out, and feet are fully washable.
The one problem if you come in shod and take off your (closed) shoes is that there may be foot odor. Going barefoot all the time isn't a problem, 'foot odor' should really be called 'shoe odor' since it's caused by bacteria breaking down the sweat in anaerobe circumstances -with little or no oxygen- rather than by the sweat itself. On a bare foot any sweat just dries up, inside a shoe foot odor may develop, and thus taking shoes off should in public should be done with a bit more forethought than being barefoot since your last shower. If you have smelly feet either leave your shoes on, or (weather allowing) leave them off all day :-)
As for whether it's bad manners to be barefoot in public, it's very unfortunate that some people confuse any alternative look with bad manners and disrespect. There aren't any real health/safety/hygiene concerns with bare feet in most places, no more so than with bare hands, and while some people don't like the sight of bare feet it's silly that the tiny strap of a flipflop would suddenly make it okay to see. While the flipflop cuts off all sensation (which is my main reason for going barefoot, feeling the ground), on *top* it still leaves the entire foot bare, silly that makes so much of a difference to those who hate seeing feet. Others associate bare feet with sloppiness and lack of personal care. But please remember not all cultures are the same and try not to take offense when none is intended. Personally I think bare feet are the most gentle and *respectful* way to walk on Mother Earth, touching the earth softly, being aware of my surroundings, making no noise and leaving hardly a print behind. Please look at the whole person before taking offense, whether someone is well-groomed and well-dressed from the ankle up or is wearing stained clothes, didn't comb their hair and was just too lazy to get dressed more than half-way.