You've had some excellent recommendations, some I would have suggested myself. I have a few more, mostly authors:
1. Dan Gutman.
2. Andrew Clements. ...both good mid-level authors, books aren't too long, but very fun.
3. Zombie Chasers series...forget the author, but there are only 2 or 3 books in the series. Silly and fun, not gory.
4. Mary Downing Hahn...ghost stories for kids. Many books, DS has only read a few.
5. Guys Read...a series of books (Funny Business is one, Thrillers is new). Short stories written by well-known kids authors. DS is getting the Thriller one for Christmas

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6. Ghost Hunters series by Cornelia Funke. Fun, silly, ghosts. What more could a kid want?
7. Warriors series by Erin Hunter. Cats living in clans in the forests, battles galore, 15 + books. The reading level is a little higher on these, but reachable.
8. You Wouldn't want to...series of history books, lots of facts, but fun and with great comic illustrations. (You Wouldn't Want to be a Civil War Soldier is DS's favorite.)
HTH. Good luck!