The Merlin Mission Magic treehouse books tend to be a little longer. Most of them read the same, but if you look at actual grade level of the books, they are rated slightly different from one another, but again, they will all be easy. My DD enjoyed them for the knowledge they presented. She particularly liked the Merlin Mission Magic Treehouse: Monday with a Mad Genius *If I recall it was about Leonardo D'Vinci... I think. Again, the research guides are interesting reads if you want to pair the two. DD read those well before she was 5 too and it drove me batty when someone would suggest she read those in 2nd grade, etc. LOL! We didn't read them all, only the ones that were of particular interest or if she was into learning about a certain topic, then we'd read that, and get the research guide etc.
All the books people are recommending are great reads! We haven't done Harry Potter yet (at 9) for the same reason people stated here, so definitely avoid that for some time!
Not sure if your child has the capability of doing AR tests at school on books but if she does, then even if she reads below her capable level, she can add a bit of a challenge (maybe) by taking the computer tests on them and strive for a goal for herself with it. I like to look books up on ARBookfinder and it tells you points books are worth and levels etc.
