Note: You can use the author studies format to study poets and songwriters as well.
Some authors, like James Herriot and Demi, write for more than one grade level. Some authors, such as Gary Paulson and Roald Dahl, could be studied during either the grammar stage or the logic stage. Other authors, such as Mark Twain, can be studied during either the logic stage or the rhetoric stage. I recommend previewing a few books by the author before deciding whether you want to use them for an author study.
Grammar Stage:
Demi (folktales) Jan Brett
Beatrix Potter (animals) Thornton Burgess (animals)
Bruce Coville Beverly Cleary
Roald Dahl Lynne Reid Banks
Jean Craighead George Avi
Dick King-Smith (animals) Lois Lowry
Marguerite Henry (horses)
Logic Stage
Eloise Jarvis McGraw Laurence Yep
Lloyd Alexander (Welsh based fantasy) Howard Pyle (historical legends)++
Madeline L'Engle (fantasty) Terry Pratchett
Geraldine McCaughrean Russell Freeman (biographies)
Susan Fletcher Rosemary Sutcliff
Louisa May Alcott Demi (biographies)
Tamora Pierce (high fantasy)
Rhetoric Stage:
Mark Twain Isaac Asimov
Ursula LeGuin (fantasy) Shakespeare
JRR Tolkien Anton Chekov (Russian plays)
Garth Nix
Charles Dickens