If You Could See Me Now by Cecelia Ahern. She also wrote P.S. I Love You, you know, the movie made everyone cry including my husband.
I was won over by the message of this book, but I have to admit I almost put it down when the second chapter was written from the point of view of an imaginary friend....almost put it down, but decided to give it a chance and I am glad I did. It isn't great literature, but it is entertaining and amusing and it makes you "believe in believing". There is a tiny bit of (predicatable) magic in the pages and it made me smile and that is enough for me these days.
On the topic of imaginary friends mine were named Poo Doo and Shoo Shoo, but I don't remember them. I think sometimes I could use an imaginary friend these days, when I'm tired of being in my own head.