A letter of acknowledgment is necessary when you receive information for which you have written.
Letter of thanks for information received
23 June 2011
Dear Mr Webster,
Thank you for your letter enclosing an account of the organization and work of your local trade association.
I am very grateful for the interest you have shown in our proposal to include details of your association in the next issue of the Trade Associations Year Book and for your trouble in providing such an interesting account of your activities. Your account is sure to inspire and encourage associations in other areas.