A few good books for 2nd graders that can be downloaded for free include classics such as "The Secret Garden" and "A Little Princess" by Frances Hodgson Burnett. These books and many others can be downloaded for free from Project Gutenberg's website Gutenberg.org.
Other great books for children available from Project Gutenberg include "The Adventures of Huckleberry Finn" by Mark Twain, "Anne of Green Gables" by L.M. Montgomery and "The Box-Car Children" by Gertrude Chandler Warner.
The website LibriVox.org has copyright-free audio books available to download, including great books for second graders such as "Robinson Crusoe Written Anew for Children" by James Baldwin.