Harmonic bonds aren't the most predictable of spells. Sunset learned this the hard way with Twilight. Involving Moon Dancer will make things either extremely complicated, or extremely simple. And that has nothing to do with harmonic bonds.
Twilight's just a little nervous. It has nothing to do with Moondancer taking over the Castle Library. Or Moondancer's smile when she suggested taking over the library. Or anything to do with libraries. It's nothing. Nothing at all.
Moondancer has always thought highly of Twilight and was devastated when she left without saying good-bye. When Twilight throws a party for Moondancer and rekindles their friendship, Moondancer decides she wants more.
It's taken a lot of time and effort, but Moondancer is finally ready to come out of her shell! Now if only she could find the confidence to ask Twilight Sparkle to come out with her...