Art that tries to be highbrow, but really isn't. Status-seeking middle class people buy middlebrow work to display, not to enjoy. Writer Virginia Woolfe famously decried middlebrow as "a sticky slime of calves-foot jelly", though her work could be classified as middlebrow too. Still, as long as you actually read the books you buy instead of just using them to set of the furniture, who cares?
My family library is so middlebrow: all leather-bound editions of the Great Works. Still, I enjoyed looking at the pictures as a kid.