The Sisters Grimm Series by Michael Buckley
The series is about two sisters who don't know they are related to the famous Brothers Grimm. There parents are missing and they've been moving from one disastrous foster house to another, until their grandma (who they never knew existed) comes to bring the girls home with her to Ferryport. The sisters (Sabrina and Daphne) learn that all the fairy tale creatures are real, and were trapped in Ferryport to prevent a war between them and the humans. That is, they are trapped in Fairyport as long as a Grimm descendant lives...
Each book has it's own adventure and conclusion while there is an overall story arc of finding their missing parents. It's fun to see the personalities and occupations Buckley gives to different fairy tale characters. These books have exciting plots, humor, suspense, and surprises. It has two strong girl characters, which makes this series unique, and all the better. You've got to read this series.
(I actually put the first book down after the first chapter or so, but later picked it up and I'm so glad I did. If you feel the same as I did, just keep reading!!)