Take a look at any character in this series who wears glasses and then stop for a moment. How do they stay on their face? The characters are drawn in such a way that the characters' ears are well below eye-level, so that the earbuds of the glasses rest well above their ears. Still, even if you ignore this (and, after all, it's also the case with Arthur, though with his, the buds rest below the ears), there's still the fact that the glasses don't have any bridge in the middle! This gives them the appearance of being two independent parts, somehow resting separately on the characters' faces.
It's the animation style, and I doubt the show's target audience would care.
Are the Book's mechanics & workings ever explained?
No, not really.
Just what is Lionel listening to on those headphones of his? And why don't Mr. and Mrs. Griff ask to at least take them off for mealtimes?
Notable: In one episode, Stanley becomes embarrassed because he thinks his ears stick out too much and wears a helmet to the dinner table. Lionel asks him what the deal with the helmet is and he retorts by asking Lionel what the deal is with the headphones. Mr. and Mrs. Griff make a new rule on the spot that neither helmets nor headphones are allowed when sitting down to eat at the dinner table. Lionel is surprisingly cool with it, removing his headphones without even seeming annoyed, but Stanley just says that he's done eating and asks to be excused. The rule doesn't seem to stick in later episodes.