The Milo and Jazz Mysteries by Lewis B. Montgomery.
The Case of the Stinky Socks.
The Case of the Poisoned Pig.
The Case of the Haunted Haunted House.
The Case of the Amazing Zelda.
The Case of the July 4th Jinx.
The Case of the Missing Moose.
The Case of the Purple Pool.
I read the seventh and most recently published book in the series, The Case of the Purple Pool, because it was one of the books nominated for the Cybils in the Early Chapter Books category. Milo and Jazz are detectives-in-training, but even with the benefit of their lessons from Dash Marlowe, Super Sleuth, the two youngsters are stumped when someone turns the neighborhood swimming pool water purple. How? Why? And will it happen again? I figured out the solution to the mystery within pages, but young readers might just have to exercise their brains to solve this one. I think mystery fans ages 6-10 will enjoy this series.

The First Kids Mysteries by Martha Freeman.
The Case of the Rock ‘N’ Roll Dog.
The Case of the Diamond Dog Collar.
10-year old Cammie and 7-year old Tessa have a very important mom and a very lively dog. Hooligan, the dog, lives up to his name and creates havoc wherever he goes. And Mom, well, Mom is the President of the United States. So Cammie and Tessa and Hooligan live in the White House with their mom and dad and Hooligan and Granny and Aunt Jen and her son, Nate, and Granny’s canary who doesn’t have a name—yet. In the Case of the Diamond Dog Collar, Hooligan receives a gift from the president’s dog in a neighboring country, and one of the twelve fake diamonds on the collar goes missing. Cammie and Tessa must put on their detective hats and go to work to find out where the (fake) diamond could be. This series is a little more challenging for readers, so I’d suggest it for ages 9-12, especially if those mystery fans are still prefer shorter books.

Young Cam Jansen Mysteries by David Adler.
Young Cam Jansen and the Dinosaur Game.
Young Cam Jansen and the Missing Cookie.
Young Cam Jansen and the Lost Tooth.
Young Cam Jansen and the Ice Skate Mystery.
Young Cam Jansen and the Baseball Mystery.
Young Cam Jansen and the Pizza Shop Mystery.
Young Cam Jansen and the Library Mystery.
Cam Jansen has a photographic memory, and that’s one of the things that makes her such a good detective. Some people nicknamed her “The Camera” because she remembers things just like a camera, and then they just called her “Cam.” These books are beginning, level two readers for very young readers. If your reader finishes these and wants more Cam Jansen, there are a slew of Cam Jansen mysteries that are in the “Early Chapter Books” category, second to fourth grade reading level.

Then, there are these classic series that still hold the attention of young readers:

The Boxcar Children series.
Encyclopedia Brown series.
Nate the Great series.