Well, if you don't need your free ebooks to be currently in copyright, check out the ones here on MobileRead, plus Project Gutenberg, ManyBooks, the Baen Free Library (
gratis but not
libre), etc. I know when I was 9 I would have gone ape over the complete
original Tom Swift series (I'd already read all the modern ones, which weren't nearly as cool) along with classics like Treasure Island, Ivanhoe, Tom Sawyer, Sherlock Holmes, and, well, tons of other stuff. That's right around when I discovered Tarzan, too, and pretty much everything Burroughs wrote is available. And Conan ... what 9-year-old (or 19-year-old, or 90-year-old) boy (or girl) wouldn't like the real REH-written Conan stories, which make all the later pastiches look lame?
That frees you from the tyranny of DRM and of the proprietary bookstores, and lets you buy any ebook reader you happen to like. As to which one that might be ... well, that's what the whole rest of this section will chime in on.
Personally, I have a Sony PRS-505 that I wouldn't trade for any other reading device out there, but it's discontinued; you might want to look at the PRS-300.