If You Could See Me Now, by Cecelia Ahern (2006, USA)

Elizabeth is a successful young woman, living in Baile na gCroìthe, a small village in the West of Ireland. Her mother left as she was 13 and she grew up her sister Saoirse. Now Saoirse is alcoholic, hardly at home, and Elizabeth is growing up her 6 years old nephew, Luke. But sb arrives. Ivan is the invisible friend of Luke. As his colleagues, he can be seen only by whom believes and needs him. And Elizabeth can see him too, although unaware of him to be an imaginary friend (nevertheless real). A strange love story that changes everything.