This is a hard question to answer because there is a ton of available material but what you want is really a curated collection and somebody has to do the curating. I can give you some ideas for specific titles based on my own reading but that will limit you to maybe a dozen titles...
- Cory Doctorow has two YA novels available for free under a Creative Commons license. Little Brother is pretty good; haven't read the other one yet. I think he even has some teaching resources for Little Brother.
- Orwell: 1984 and Animal Farm are freely available and I remember at least one of them being on the curriculum when I was in school. 1984 would be a great one to study along with Big Brother...
- John Scalzi has a freebie
here that is very good and kids would enjoy it. Fun one for a free choice read.
- You might consider teaching a parent volunteer how to do some basic HTML formatting to save Wikipedia articles as proper ebooks kids could download and read on devices...