This document discusses different types of important business letters, including letters of inquiry and order, collection letters, and letters of complaint. It provides guidance on writing each type of letter for both customers and sellers. For inquiries, it notes the information a customer should include and how a seller should promptly acknowledge. For orders, it outlines what a customer's letter should contain and how a seller should quickly respond or explain any issues. It also discusses different types of orders. For complaints, it provides tips on what details a letter should include and how suppliers should deal with complaints politely. Finally, it discusses collection letters and the different stages in a series of collection letters to tactfully request past due payments.