What is a Middle Reader?

Well, it depends on whom you ask, but there are a few general guidelines:

The age range is typically considered 8 to 12; however, different age ranges do appear (i.e. 9 to 12). The best rule of thumb is 12 and under.
If the age range is included:

Hardcovers note it on the front flap of the dust jacket.
Softcovers note it on the back of the book.

The main character tends to be 12 or younger.