These are the books I use in my mystery unit. I use them for small book club discussions.
The Mystery of the Dark Lighthouse
The Dollhouse Murders (most kids really like this one, because it's a bit creepy!
Who Stole the Wizard of Oz? (used for lower level readers)
The Chameleon Wore Chartreuse (Great humor for higher level thinkers! My group this year has LOVED them and want me to buy more in the series)
Okay, there are a few more, but I'm going blank! If I can think of them, I'll send them to you.
Oh, my kids typically love the Encyclopedia Brown series. They like having the ability to try and figure it out on their own before they read the solution in the back.