Define
A letter in one person's voice, written as if to one reader, signed by the person who owns the outcome.
Mechanism
Personal authority. The signature is the proof. The tone is the brand. A committee cannot write this and keep the mechanism.
Verdict
Trust sits on a person, or the company is asking for a relationship rather than a transaction.
Don't
The buyer wants a spec sheet, or the founder voice is a costume. Forged intimacy is worse than no letter.
Copy skeleton
Dear [READER], [Why this letter exists: the stake] [What was seen / built] [What is offered] [The promise] [Signature / role]
Gulf note
If the letter is from a founder, it must sound like that founder. Ghostwritten polish is the tell. In Arabic, sign the name people already use in the room.
Rights-cleared examples
Wahrhaffte in Grund gelegte Statt Passaw
72 Master Letters and What Made Them Pay
Advanced Business Correspondence
Alphabets Old and New, for the Use of Craftsmen
Applied Business Correspondence
Business Correspondence Library, Vol. I: How to Write the Business Letter
Business Correspondence Library, Vol. II: How to Get and Hold Business by Letter
Business Letters, How to Write Them
Business Writing: Articles, House Organs, Reports, Advertisements
Effective Business Letters: Their Requirements and Preparation
Examples of Modern Alphabets, Plain and Ornamental
Hand-book of Modern Business Correspondence