The Project Gutenberg wiki catalogs more than 53,000 free ebooks. They are available in epub, pdf, and popular ereader formats. The books are out of copyright in the US, so are free to distribute and copy as you like. Because they are out of copyright, these are mostly older books published before 1923. But they still include some popular timeless children’s authors such as Beatrix Potter, Lewis Carroll, and Mark Twain.
Three of book categories to look at for K-12 are: Most Downloaded Books, Top 100 Books and Authors, and Children’s Bookshelf. The Project Gutenberg collection is now available as an app for Android, iPhone/iPad, and Kindle ereaders.