Project Fairy by Jacqueline Wilson

Mab's mum is obsessed with fairies - she even named Mab after the Fairy Queen! Their flat is full of fairy ornaments, tiny fairy furniture and they've even got fairy lights in the toilet. Mab doesn't quite get it, but she knows that fairies make her mum happy, especially after Dad left.
When Mab comes into school with a new fairy dress on her birthday, her teacher gives her a book all about Victorian fairies. She reads it with her mum and her little brother Robin, and she's surprised to see the drawings inside are nothing like the sweet fairies she imagined.
But the biggest surprise of all is a tiny thing that tumbles out of the pages of the book...