Two series my nephew enjoyed: Ursula LeGuin's Catwings (about cats who were born with wings; I think there are 4 books in the series), and Gertrude Chandler Warner's Boxcar Children.
We were doing the original Oz books for awhile, but got stymied because the library only had 1 copy of Dorothy and the Wizard in Oz and it was checked out...I was pretty Oz-ed out by then anyway, as it was book 4. So we took a break to read Wind in the Willows. I'm reading to 2 girls, ages 7 and 3, BTW, and WitW was definitely above the 3yo's level.
Mouse and the Motorcycle, or other Ralph Mouse books by Beverly Cleary.
Mrs. Frisby and the Rats of NIMH (dh loved parts of this series, and kept reading ahead).
Also, ask the children's librarian at your local public library. The librarians at our library are *wonderful* at coming up with books for us, or else pointing me to cool indexes which group books every which way.