Rating - 2: not worth reading (skip it)
Not so much a book as a puzzle, and a poorly defined one at that. This books serves mostly as an inducement to buy The End, without which the puzzle makes little sense.
Letters to Beatrice. Letters from Beatrice. Letters re: Beatrice. This is a supplement to A Series of Unfortunate Events, addressing Lemony Snicket himself and Beatrice(s).
An epistolary novel would combine the letters into a coherent storyline. This being A Series of Unfortunate Events, that does not happen. They letters do not form much coherent at all except anagrams.
There are many anagrams. If you like them, you will find them. If it does not occur to you to anagram words, the obvious one will be the only one you are likely to find. The book comes with punch-out letters in case you missed the pattern of letters (ABC) in letters (Dear sir), but other things like names and "baticeer" embrace anagramming. The letter letters anagram to more than one thing, by the way, all of which have multiple implications or interpretations.
So basically, that is a rorschach.
This book reads differently before and after The End, which is to say it hints at some things in the later book and makes less sense until you have read both books. Even if you have read The End, I would not recommend this book. It does not add anything significant to the storyline, and what it does add is shadowy.
The book does contain some great bits of Lemony Snicket writing, as all the books do. It picks up some items from Snicket's self-referential statements throughout the book, most notably the 200-page rejection letter he once received as a suitor (carried by a pigeon). Root beer features, but sugar bowls do not.