For the kids:

Nurk : the strange, surprising adventures of a (somewhat) brave shrew is a tale of an accidental adventurer who reads his grandmother's journal for inspiration.

Dear Mr. Henshaw is a series of letters a 5th grade boy writes to an author about the complications in his young life.
Fiction:
Which Brings Me to You is the communications between two people drawn to each other at a wedding. They decide to get to know each other better before starting a relationship, and we get to read all their letters and emails!
We Need to Talk About Kevin is a series of letters written by Kevin's mom to

Non-Fiction:

For more books like this, search the library's catalog under subject for "letters fiction" or "epistolary". Or ask at the Reference Desk!