Marthe Jocelyn 
Invisible Enemy 

Alyssa is mean, sarcastic, and just plain no fun. Billie Stoner considers her to be an enemy and sometimes wishes that Alyssa would just disappear. Thats exactly what happens when Alyssa snoops in Billies knapsack and accidentally makes herself invisible with Billies secret powder. Now its up to Billie to help somebody who has made her school life a total misery. Will she be able to save Alyssa? Does she even want to?Young readers, who were delighted by Billie Stoners adventures in The Invisible Day and The Invisible Harry, will be thrilled to see nasty Alyssa get her comeuppance and Billie make an improbable new friend.From the Hardcover edition.