It's not generally necessary to reply to letters of appreciation, but when these letters are coming from other businesses, or professional colleagues, a response is sometimes appropriate. This is a professional business courtesy.
Note: These letters of appreciation are also good indicators of high performance by people in your business. Always keep track of who's being complimented for what.
Dear (use full title of the addressee)
Letter of appreciation for services (insert specifics)
I refer to your kind letter expressing your appreciation for services rendered by (insert title and name) / our business / firm / company.
I'd like to thank you for taking the time to write this letter. Your letter has been passed on to (title and name).