This kind of letter can be used for thank-you letters, reply to requests, complaints etc.
In this type of letters you are expected to use: fomal linking words, words/phrases in the passive form, have a respectful, polite tone and in general use some formal language.
You should begin you letter with Dear Mr.../ Dear Mrs...
In the first paragraph, the writer should explain the reason for writing.
In the main body paragraphs, should express thanks or gratitude or reasons for the complain (depending on the kind of letter you are writing.)
In the conclusion, the person should thank again or ask to have his/ her situation resolved. (again depending on the kind of letter you are writing.)
In the end you should use Best regards, or Best wishes, and sign.
At this stage we will work with your book (page 92) and workbook (page 49).