P.S. Longer Letter Later, by Paula Danzinger and Ann Martin

When Tara moves away to Ohio, she and Elizabeth are forced to carry on a correspondence through letters, and this book reproduces the first year of those letters. With each author penning one of the girl's letters, the story of an exciting year with various family traumas comes out one letter at a time.

It's a pretty cheapo way to write a book: writing pretend letters back and forth, with one author not quite knowing where the other one will take the story. The trouble is that it's more of a writing exercise than a coherent book. And the story is a big yawn too -- coming out in letters, we never get too see much of what is going on in the character's hearts. Superficial fluff.