I liked this episode for the same reason I liked the castle-exploration episode. Character interactions. Marvelous character interactions. The jokes were fun and came at a steady pace. We got a marvelous bit of world building for those who bother to look, what with the two-headed dog, the Discord lamps, the "wheelchair..."
I think one of my favorite parts was Spike and that colt trading their mint copies like it was a hostage exchange.
I grinned like an idiot when Rainbow Dash fainted. And basically every time she was onscreen. I do a lot of idiot grinning when it comes to My Little Pony.
I think we can unequivocally settle that Twilight Sparkle's library is in fact her private library. Seems she lets ponies borrow every now and again, but it isn't necessarily her job in the canon (not that I wouldn't read any story where that was part of the plot--it's an idea with potential).
Buuuut... the library was full of books before she arrived. Shucks. Nothing unequivocal about that. Maybe she was just running out of room for her personal book collection, which is separate from the library? I dunno anymore.
A nice addition to season four, and one that I'll happily come back to for a second viewing once the season is through (and on Netflix).