Charlie was one of the 2 teenage girls killed by a train as they were setting off for Cambridge last year - her father has been writing letters to her ever since and they have just been published.
There was an extract in The Observer last Sunday (I started reading it at work and just had to stop or else I would have been howling down the phone next time it rang). He was on Woman's Hour this week talking very movingly about Charlie, and the book is Book of the Week on R4 starting tomorrow.
Don't even think about reading/listening if you'll need to function immediately afterwards.