There's this ad running on NPR at the moment, both in their audio programs and on the website that looks a little something like this:
The audio ad also says pretty much the same thing:
a [...] middle-grade adventure
It sounds awful as ad lingo, especially if you compare it to ads that talk about good things being high-grade.
I'm sure this has to be some writers vernacular though, and I'm guessing they wouldn't advertise their book as mediocre.
What does middle-grade refer to? and when did this terminology come about?