Marginaliens by Philip Gross

I haven't finished this book yet but I'll tell you what has happened so far...

A girl called Enna likes to doodle. So one day, in class, her teacher tells her to draw what she sees. Enna draws a scruffy looking creature which she calls a gargoyle because she is in a bad mood. When she gets home, she adds arms and legs to it. Enna then takes a rest but when she looks back at her drawing, the gargoyle she had drawn, had moved and changed direction. She  gets so scared she throws the book into her toy cupboard and covers it with her duvet. A few days later, Enna's little brother of the age 7 disappears. 

But only Enna knows where he is - in her book, in her drawing...