The Ghostwriter Secret — Mac Barnett

Steve Brixton is America's next great kid detective (and now he has his own agency to prove it)!
When twelve-year-old detective Steve Brixton gets a call that the Fairview diamond has gone missing, there are only two things to do: grab his new business cards and take the case! But Steve quickly learns there's an even bigger mystery afoot. MacArthur Bart, a.k.a. Steve's hero, a.k.a. the author of the Bailey Brothers Mystery series, has been kidnapped! With the man who taught him everything he knows about sleuthing in trouble, Steve and his best chum, Dana, know they must stage a daring rescue...

Once they find out where Bart is...

And who took him...

And why...