The Westing Game by Ellen Raskin.
The mysterious death of an eccentric millionaire brings together an unlikely assortment of heirs who must uncover the circumstances of his death before they can claim their inheritance.
The Nancy Drew series by Carolyn Keene, is another set of classic mysteries. There are over 175 books, which you really don’t have to read in order. If you want to really start at the beginning, try, The Secret of the Old Clock, in which Nancy, unaided, seeks to find a missing will, and the search not only tests her keen mind but also leads her into a thrilling adventure. There are more modern versions of Nancy, also by Keene, such as Without a Trace. Here, Nancy takes the case when a very old and valuable Faberge egg disappears.
Chasing Vermeer, by Blue Balliett.
When seemingly unrelated and strange events start to happen and a
precious Vermeer painting disappears, eleven-year-olds Petra and Calder combine their talents to solve an international art scandal.
Hoot, by Carl Hiaasen
Roy, who is new to his small Florida community, becomes involved in another boy's attempt to save a colony of burrowing owls from a proposed construction site. But who is the barefoot boy, and who is vandalizing the construction site?
The Sky Inside, by Clare B. Dunkle
This is a very new title – a sci-fi, futuristic dystopia and a great mystery!
Martin lives in a "perfect world" under the protective dome of suburb HM1, where every year a new generation of genetically-engineered children is shipped out to meet their parents. And it's all about to come crashing down because a stranger has come to take away all the little children, including Martin's sister, Cassie, and no one wants to talk about where they have gone. Martin has a choice either to remain in the dubious safety of HM1, or to break out of the suburb into the mysterious land outside.