Fun mystery, but might be a little scary for really young kids

The Book of Lost Things - Cynthia Voigt, Iacopo Bruno

Mister Max was well written and fun for middle grade children (8+).  The mysteries are interesting without being too easy or obvious or too hard for kids to follow.

 

The only caution I'd have about this book is for younger kids - it might be kind of scary for them because the parents disappear and we have no idea if they've been kidnapped or have run off and abandoned Max, both of which I think could scare a younger kid if they have separation anxiety with their parents already.