Kay, I'm sure your sweet Addi-girl would never act the way Adelaide Addison did. (More on Adelaide in a moment.)
I've read some books where the letter-writing style crashes and burns, even some presented as e-mails (of which I can't think of a single example that I enjoyed). I think the real advantage in this novel is the many and varied personalities and points of view, each of whose characters was well-defined and showed through in his or her correspondence. I could almost hear each person talking as I read. Even the vocabulary level changed from person to person. It was handy having the date and to/from info at the top of each letter, but I think I could have figured it out without that.
Another strength is that it seemed much more natural for the characters to reminisce about past events in letters. I think it would have seemed much more forced in conversations.
Now back to Adelaide Addison for just a moment . . . I have read a couple of criticisms that the book is anti-Christian because Adelaide was portrayed as a narrow-minded unforgiving Bible thumper. I am a Christian and that did not strike me at all. I found her to be a crotchety old lady, and I thought that her character would be redeemed at the end (but sadly wasn't), but I did not think the whole novel was anti-Christian because of one character. I know of at least a couple of people like that in my home congregation.
Sara