I have been a fan of this genre for a considerable period having enjoyed, in particular, the Timewaster Letters series and the "Daddy": The Henry Root letters.
The Morello Letters, in my opinion, take the genre to a whole new level.
The letters taken individually are 'laugh out loud" funny. Indeed, I had an embarrassing moment at a tube station when I went into uncontrollable hysterics whilst reading (my personal favourite) a letter to the Health Secretary, Patricia Hewitt, made even funnier by a humourless, dead-pan response.
However, what takes these letters onto a new plane is the careful development of the Morello characters through the letters. And the Monty-esque, off-the-wall use of farmyard and zoo animals as a means to hysterical humour.
I really cannot recommend this book highly enough. It is a must for anyone with a sense of humour!