I am not sure what you mean by service pledge letters?
If you are thinking of offering customers guarantees for the service they receive you should read "Extraordinary Guarantees" by Prof. Christopher Hart - ISBN Number 0-8144-5064-4. Chris Hart is an adjunct professor ar the University of Michigan and probably the world's expert at service guarantees.
Although he's US based he has worked in the UK as well and helped Travel Inn develop their service guarantee. For a more detaied reading list visit my website.