Maybe Secret series. Name of this Book is Secret by Pseudonymous Bosch is the first one.

Grace Lin has got some fantastic books. Where the Mountain Meets the Moon is set to become a classic and is an excellent fantasy adventure story.

There's the Horrible Histories, Horrible Science, Horrible Geography series too.

You might try if you haven't done so already - Wind in the Willows, Pooh books (they're lovely, innocent, well written, and classics!), Doctor Doolittle, The Little Prince, Alice in Wonderland, Carry On Mr. Bowditch - though an unabridged Wind in the Willows and Alice in Wonderland might be too advanced at the moment.