Just finished an author business letter unit with 7th grade classes. I've always enjoyed the authentic nature of the business letter writing unit because we really mail the letters out. (Yes, some have received written responses along with free coupons, free tote bags, autographed photo, etc.)
The author letter is new for me: Students chose an author whose book they enjoyed reading this past year. They read several model author letters from a book I borrowed from colleague, studied the elements of a good author business letter, and crafted their own. (One additional resource was brainpop.com - schools can obtain free membership - instructional cartoon videos, many games, quizzes, fun!)
I'm very happy so far with the personal connections they made and expressed in these letters. For example, one girl wrote that the book inspired her and her friend to do something unconventional at school - wear dance costume (tutu skirts) over regular clothes - just for fun! Several described sharing the books with friends and siblings. One candid girl said, "I don't mean to be rude but I never heard of you before my grandmother gave me your book." Another connected the Holocaust information she learned from Maus to what her mother was currently studying in college.
I've assigned several "reading letters" during the year, in which students wrote to ME about their independent reading books, connections, evaluations, etc. Overall I found their reading letters (to me) formal, generic, and just not that interesting. I think the authentic audience feature of these author letters pushed their thinking and writing to the next level (up)!