Percy Jackson and the Olympians (book 6). A contemporary American take on Greek mythology, and one of the top bestselling series for 9-12 year olds of all time.
Younger middle grade readers should definitely start at the beginning of the series, with The Lightning Thief. In that book the protagonist is 12 years old. Percy and his friends age between each book.
In his interview on The Best Rick Riordan Books,13 year old Alex explains why he finds the Percy Jackson books, and others by the same author, so appealing.