I got matched with a student for the One Book At a Time project @jackiback posted about a few months ago so I'm trying to work up a list for the year. Thank you!
Jake Maddox writes sports related books that are really popular at my school.
Basketball titles: Point guard prank, Free throw, Hoop hotshot, Slam dunk shoes. He also has a Team Jake Maddox series with a few basketball titles.
Not mystery but the Ascendance triliogy, The False Prince and The Runaway King are really good. {The Shadow Throne release Feb 2014.} Sage is smart, sacrastic, and I just loved him. There enough action, adventure and a bit of "mystery" that'll keep the readers attention.
Eta: thought of a few more... Crime Files (forensic mysteries), 39 Clues, A to Z Mysteries and Hardy Boy (believe it or not) get checked out pretty regularly.
Re: Recommendations for a 6th grader?