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 Jan Brett
Beatrix Potter Thornton Burgess
Bruce Coville Beverly Cleary
Roald Dahl Lynne Reid Banks
Jean Craighead George
Dick King Smith
Logic Stage
Eloise Jarvis McGraw Laurence Yep
Lloyd Alexander Howard Pyle
Madeline L'Engle Terry Pratchett
Geraldine McCaughrean Russell Freeman
Susan Fletcher Rosemary Sutcliff
Louisa May Alcott
Rhetoric Stage:
Mark Twain Isaac Asimov
Ursula LeGuin
JRR Tolkien
Garth Nix
Charles Dickens