There are, as the others have said, a TON of "tweener" books out there. I would recommend anything by Beverly Cleary (Ribsy; Ramona the Pest) or Gary Paulson (Hatchet; Canyons). Nancy Drew and The Hardy Boys are also good, as are The Indian in the Cupboard, Just Ella, and Catherine Called Birdy. The Artemis Fowl series is another really good one, along with The Alex Rider Adventures by Anthony Horowitz (kind-of like a 12-13 year old James Bond), or really anything by him, and books by Elizabeth Haydon such as The Floating Island. Another favorite series is the Eragon series, which was originally written by a young boy no more than 15, and of course the Harry Potter series, maybe just the first few books around that age- they get progressively darker as Harry gets older. Goosebumps is also a good series, by R.L. Stein, as well as Animorphs for around 11-12. Others you might like are the books by Rudyard Kipling, such as The Yearling. It's a bit deeper, but often on 7th and 8th grade reading lists- closer to 12 than 9 though. Greek and Egyptian mythology is also often studied and found intriguing around 12. But one of my favorite series for kids around 5th grade is the Redwall series. I think everyone should read them- excellent stories, great characters, amazing heroes, and very conniving villains who never win. And the best part- all the characters are animals! Hope this helps!