Feeling Sorry for Celia

Blurb

Feeling Sorry for Celia is a young adult novel by Jaclyn Moriarty. It was first published in 2000 by Pan Macmillan. The story is told in a series of letters.
Life is pretty complicated for Elizabeth Clarry. Her best friend Celia keeps disappearing, her absent father suddenly reappears, and her communication with her mother consists entirely of wacky notes left on the fridge. On top of everything else, because her English teacher wants to rekindle the "Joy of the Envelope," a Complete and Utter Stranger knows more about Elizabeth than anyone else.
But Elizabeth is on the verge of some major changes. She may lose her best friend, find a wonderful new friend, kiss the sexiest guy alive, and run in a marathon. So much can happen in the time it takes to write a letter...