It is a part of the history of the book. But it probably wouldn't add to the price you get. You find the odd letter in proofs and review copies. It is usually an attempt by the publisher to get a bit of free advertising if the recipient writes a review and it is published.

. ------------------------------------------------------------------------------------------------------------------------------------------------------Whomever you may murder, there's no police like Holmes.
Σ Ψ Δ
σ ψ δ