Chick-sf: Mindy Schanback, Princess From Another Planet, (New York: Holliday House, 2005).
The book has a pink cover and a tiara. It's a very pink book. And I loved it.
There isn't much to say: the book is very written--I couldn't put it down--and is not dissimilar to the Princess Diaries but with the twist that Gracie Wright thinks her mother is mad, because her mother claims to be a Princess of Pannadeau and insists on teaching her how to kill Maluxziads.
It quickly turns into a romp/screwball comedy, triggered in part when Gracie swaps places with her richer cousin so she can go to riding camp (the cousin is bookish: I look in vain for nerds-as-hero in modern YA sf, despite their ubiquity in the older juveniles).
Gracie defeats the invaders, discovers her mom is not insane, and life goes on. The only sadness is that the wormhole through which her mother and the invaders travelled closes at the end of the book--ending of any chances of long term consequence. A "sealed" story in which all the "future" is personal, not sf-nal.