If you've had enough of Nancy Drew, Hardy Boys, or Trixie Belden, try these books on for size:
Andrew Clements is a widely popular children's author; his books are centered around realistic school situations, with a great balance of humor and thought-provoking subjects. Room One: A Mystery or Two is not only an entertaining mystery, but it also addresses such issues as the decline of rural towns, including rural homelessness and rural education. Clements doesn't preach, and the issues never overtake the story. If you need a quick read, Room One is an excellent choice.
The Westing Game won the Newbery Medal in 1979. A recent reread proved that, save for a few outdated terms ("Oriental" to describe a Chinese character and "Mongoloid" to describe a mentally challenged character), the book's humor and surprising turn of events should definitely hold the interest of young mystery fans. The storyline involving a group of characters brought together in order to discover the murderer among them creates a fascinating study of characters.
M.T. Anderson is one of modern children's literature's funniest and wackiest authors. The Clue of the Linoleum Lederhosen is funny and fast-paced enough to appeal to most readers, but readers familiar with the old-school formulaic children's mystery series such as Nancy Drew, the Hardy Boys, the Bobbsey Twins, the Boxcar Children, and Trixie Belden will definitely get the most enjoyment from the book, as this is a spoof on those characters. If you're looking for a purely fun and hilarious read, this is your book.
If you're looking for more ideas, check out the Edgar Awards, which have divisions for juvenile and young adult novels.