It isn't in letter form, but have you looked at "The Curious Incident of hte Dog in the Nighttime" ?? It's in a diary format and it blew my mind when I read it the first time through. The play is a work of art too.

Also, I agree that Guernsey and Books were characters in TGL&PPPS. It reminded me a little bit of when when authors use quotes from other books as chapter titles and you gleam some advance warning about what is coming. I love that