Key Hunters is a new series by Eric Luper that's a good choice for older elementary kids not yet ready for fantasy door stopper books. The font size and spacing fits my sense of "chapter books for young readers," and the plots are such as will be more pleasing to the young reader who has little experience with genre fiction, for whom mystery solving in historical times and spy foiling with lots of technology involved are still new fictional ground. The first two books, The Case of the Mysterious Moonstone, and The Spy's Secret, just came out (Scholastic, April 26, 2016).
But though the moonstone mystery is solved, Cleo and Evan don't find the good librarian. So they head into another book, The Spy's Secret, where they are spy kids with lots of gadgets, trying to foil the plot of the evil Viper who's plotting world domination from his underwater lair. The bad librarian is also there a villain, but somewhat more ambiguously so than she was in the first book, which adds interest. Once again, they don't find the good librarian, but perhaps they will in the third book, The Haunted Howl, coming in September.
If you are older than ten or so, you can skip these, but they are good ones for the target audience. These are books I'd give to any fans of the A-Z mysteries, for instance (do kids these days still read those?). The many cartoonish illustrations will help uncertain readers along, and the stories move briskly. And of course they are good gateways to all the books for older kids out there in which the characters fall into the world of stories; for instance, a nice next step might be The Island of Dr. Libris, or Chris Colfer's Land of Stories series, though the later, in particular, is a perhaps a bit of a jump in terms of number of words....
Here's what I personally appreciated--Evan and Cleo work together as equal friendly partners, and as the cover shows, Evan's a kid of color, a fact that has no bearing on the plot.
disclaimer: review copy received from the publisher