This book is centred around a protagonist in her 30s - all the main characters are middle aged or older - some significantly older. If nothing else, it's really refreshing to have a fantasy book that isn't about a teenager.
It was a bit slow to get into at first - understandable given that it is epistolary - but gets interesting once the letters start to revolve more around the happenings with the letters. It does get a bit hard to read towards the end with all he substitutions, but worth it. On an unrelated note, writing a sentence using all letters of the alphabet in 35 letters or less isn't that hard if you're willing to be completely unhinged about it.