Just saw this thread. These audiobook suggestions are so useful!
In addition to classic and modern fiction, there are some other genres that can be great audiobooks. Some things my 1st grader has enjoyed in audio format include:
Plays (e.g. Shakespeare's comedies or The Importance of Being Earnest by Oscar Wilde)
Poetry read aloud by the author (e.g. Robert Frost)
Seamus Heaney's translation of Beowulf (read aloud by Seamus Heaney himself with his great accent)
Also, how about non-fiction books? For example:
Blizzard: The Storm That Changed America (science and history)
I would not recommend Lord of the Flies; this is truly grim and haunting. Tom Sawyer is great but I would also recommend Huck Finn as a book best discussed with an adult (e.g. explaining the use of the word "nigger" and complex views of society). I think Around the World in Eighty Days would be great for this age.
