What is a must-read book?
Magic Treehouse, Road Dahl books, and Harry Potter are must read books in the sense that they seem to be a magic recipe for creating students with excellent English.
If I were to suggest a book that is not well known and a little more unique, I would suggest books by Randall Munroe. These would be books for more advanced students, but he writes about science in a super creative way that will make your child into a genius.
Kindergarten: "Where the Wild Things Are" or anything Arnold Lobel, especially "Frog and Toad" series.
First Grade: The Dictionary (It sounds crazy, but students at this age are so curious, they'll read the dictionary on their own)
Second Grade: "Charlie and the Chocolate Factory" or anything Roald Dahl
Third Grade: The Fudge series by Judy Blume
Fourth Grade: A book about a personal hero. If they don't have a personal hero, introduce them to a history book about a country or period of time they are interested in. They'll find one.
Fifth Grade: "The Giver" by Lois Lowry
Sixth Grade: Have them read the first paragraph of any random book they pick up from the library and when they find one they like, take it home.