The Common Core State Standards website provides hints on what your class should be reading. Take a look at Reading: Literature Standard 9. What does this mean for the middle grades?
Third Grade (RL.3.9) - Compare and contrast the themes, settings, and plots of stories written by the same author about the same or similar characters (e.g. in books from a series).
Fourth Grade (RL.4.9) - Compare and contrast the treatment of similar themes and topics (e.g., opposition of good and evil) and patterns of events (e.g., the quest) in stories, myths, and traditional literature from different cultures.
Fifth Grade (RL.5.9) - Compare and contrast stories in the same genre (e.g., mysteries and adventure stories) on their approaches to similar themes and topics.
Sixth Grade (RL.6.9) - Compare and contrast texts in different forms or genres (e.g., stories and poems, historical novels and fantasy stories) in terms of their approaches to similar themes and topics.
Clearly, the CCSS encourage third grade teachers to include author studies in their literature choices. Fourth grade teachers should go heavy on folklore, including mythology. In the fifth grade, genre studies are suggested. Sixth grade teachers would be smart to include units that compare and contrast stories, poems, and dramas, as well as those that take a look at the similarities and differences of different genres.
Since I teach fourth grade, I am in the process of developing a strong folklore unit. Each part of the unit needs to support students' understanding of folklore, stories of the people. At the same time, standards must be tightly woven into the fabric of the unit. My first attempt involved comparing Cinderella stories (folklore and parody), including the novel Ella Enchanted. A document entitled Ella Enchanted Grade 4 Overview, free on Teachers pay Teachers, describes my grand experiment: addressing nearly all of the Reading: Literature standards in one unit. Aesop's fables are featured in two sets of lesson plans: Finding a Theme and Summarizing and Writing Fables. I am currently working on materials related to mythology and Kipling's Just So Stories.
For more guidance, you can look deeper into the Common Core State Standards Initiative website. Tables list Range of Text Types for K-5 and Range of Text Types for 6-12, as well as texts that illustrate complexity, quality, and range of student reading for K-5 and 6-12.
Since today is St. Patrick's Day, I feel compelled to quote the Irish writer, Oscar Wilde: "The only thing to do with good advice is to pass it on. It is never of any use to oneself." I hope this information helps you in your quest to select the best literature for your classroom!