I would beg to differ with this opinion. Unionization does not always make for the best employers. The conditions might be better and the pay might be better, but the boss and the fellow employees may be completely miserable and bent on making everyone else miserable too. There isn't enough pay in the universe for me to put up with a boss who yells at me.
The question was about the best libraries to work for. Bosses and fellow employees come and go. Unions have nothing to do with creating better bosses; unions are they because the default condition of bosses is negative. Without a union, you are at the mercy of the boss and the boss's boss.
But basically, I'm talking about places where as a full member of the faculty you work 8 months a year for a year's pay and spend only a third of your time on the reference desk. You can have a life like that with a bad boss or you can work 50 weeks a year with no job security and still have a bad boss. And then there's the money.