This was one of my absolute favorites growing up, and it's a short, powerful series that readers will love.  This series is written by CLAMP, a group of female mangaka who have also written CardCaptor Sakura, Wish, Clover, Chobits, xxxHolic, Tsubasa, and many others.  MKR is an Isekai story that follows three junior high schoolers who are magically transported to the crumbling planet of Cephiro, and they must travel the land seeking the blessings of three gods in order to save the planet from the evil Zagato.  Zagato has imprisoned the Pillar of Cephiro, limiting her ability to pray for the planet and its people, causing its destruction.  In  Cephiro, one's will becomes reality, and the girls use this to their advantage as they gather power to defeat Zagato and his horde of followers.  There's something here for everyone - magic armor that evolves as they do, fluffy cute creatures that sold extensibly as real-life plushies, brooding, mysterious gentlemen, elaborate costumes, and more.  If your readers like these three volumes, chances are they'll also enjoy Magic Knight Rayearth 2, the three-issue series dealing with the fallout of the end events from MKR 1. 

It's almost like Viz figured out that librarians are buying their books and we have very little guidance as to what's good...  Oh wait! they did! Here's the summer edition of their Manga for Libraries publication, and there's a LOT to go through!