Growing up, my father spent a decent amount of time travelling abroad. He was in the Air Force, and would have to go on temporary duty for a few weeks or months (twice, he had to leave for a year at a time). This was all before the internet was a big deal and easily available, so we communicated by written letter. The memory of coming home and finding a new letter waiting for me is one that I remember fondly, and I've always had a fondness for snail mail. When I found out that two authors had written a series of YA novels via letter, I knew I wanted to read them, and I'm glad that I did.
The first book in the series, Sorcery & Cecelia: or The Enchanted Teapot, introduces two characters as good friends who are spending some time in different parts of the country. They are writing letters to each other, describing what's going on in their respective territories and asking questions about the other's predicament. Even though the tale is told only through letters from the two main characters, there's a wonderful amount of character development and world building.
I love that the authors wrote these letters without discussing the plot with each other, so each letter they received revealed something new about the characters, and referenced past acts that they might not have known about. The series goes on for three books, and they each have the charm and humor of the first.