Shugo Chara!, Vol. 2: Friends in Need

This is mostly a “laying the groundwork” book. It introduces the hierarchy of Easter, the bad guy group for which Ikuko is a low level member. It also introduces concepts that will come up frequently for the rest of the story. But not a whole lot actually happens. The Guardians bond. There's some innuendo that makes it clear why this manga about elementary school kids gets a “teen” rating. And there's a cliffhanger ending that indicates volume 3 is going to be a whole lot cooler (and you'll probably want it on hand so you can continue reading immediately.)