depends
What author you pick depends on what genre you are teaching.
If you're doing small moments, Angela Johnson, Ezra Jack Keats, and Donald Crews are recommended. I have done all three and like parts of each of them.
We added a realistic fiction unit for 2nd and we used Kevin Henkes as our mentor because although the stories have talking mice, they are truly the clearest examples of realistic fiction with "real" problems kids face (being teased, missing home, getting lost, etc).
I have heard of teachers using Leo Leonni for a unit on teaching fictional story elements (problem/solution/lesson).
For persuasive writing, you can use Dear Mrs. Larue, etc.
If you're doing a unit on Writing for Readers (writing for clarity, conventions, and purpose) you can use really any talented writer of shorter, simpler texts- Cynthia Rylant easy readers.
Go to this website for more specific skills!
http://www.baldwin.k12.mi.us/filesection/696/Mentor Book List.pdf