Kevin Heinkes is a good one to use- he has a collection of small books called "A Box of Treats" which have a lot to work with. We also did Angela Johnson, and got a few of her books through Scholastic. Scholastic used to have a "Lucy Calkin's Set" of books to use with the Units of Study, but you'd have to talk to a representative. I got them a few years ago- 12 books for about 15 dollars.
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.
Joshua's Night Whispers is the suggested one, so it is an easy place to start, although there are definitely many others. I ordered mine from Scholastic in a kit that was mentioned in a previous post, but before I got my own copy, I would check it out from my local library (it's a board book). I also found a great easy reading book from the Rookie Reader series Level A that my library has called Luna the Wake-Up cat and it is very similar to Joshua's night whispers and uses ellipses, lists, and repeating lines. It has a nice benefit as Joshua's Night Whispers does of being really simple. I also read Short Cut, it demonstrates all the techniques discussed in the unit. Owl Moon works well too. I also pulled in Seabiscuit by Megan McCarthy. Even though it isn't a small moment story, it works great for the lesson on research details. After we read the story, we watched a short clip on you tube of Seabiscuit's actual race described in the book and it was almost word for word just like the end of the book. My kids were really impressed. You could see the 'wheels turning'.
I am using Ezra Jack Keats right now as our mentor author for small moment stories. We read many of his books earlier in the year, so it was very easy to come back to as the stories were already familiar to the students.