Reading is not a favorite for my son. He struggles with it. BUT he loves to listen to a good book. It can be read out loud or even on a CD. He can get lost in a story for hours on end. I know in my heart his reading ability will improve one day but until then at least he can still enjoy good books by me reading them or him listening to them on line. He loves books with adventure, dragons, surprise and we just learned about stories about homeschoolers. There are so many books that are set in a school setting it is nice to read one that is not but still about daily life that is not during the summer or during a holiday break from school. It is great when we can use literature based word list to help improve not only vocabulary understanding but also the spelling of said vocabulary words.
I mentioned audio books on CD. We listen to a lot of them from the library. And when my son is not grounded from his mp3 player he usually has one or two stories on his mp3 player. I have found a lot of nice free books on booksshouldbefree.com and AudioBooksForFree.
If you want to purchase books there are plenty of sites out there for that. ChristianAudio has a free download every month. Granted they are usually for an adult. But if I chose to pay for audio books I would go with this site as they are a christian site.
I do warn you not to download any books for your child that you have not listen/read yourself. Just because the name and the description sounds OK there still might be things in the book you do not agree w/ or want your child listening too.