The Rithmatist (Dragonsteel Entertainment/Tor, 2013) by Brandon Sanderson was one of those lucky finds off the library shelf. It has an interesting premise and surprising plot twists. Aimed at the YA market, as the main character is 16, it's definitely appropriate for upper middle grade readers who like Harry Potter.
Rithmatists--those who can make chalk drawings come to life to fight the wild chalklings in far off Nebrask--are set apart in school society. Poor scholarship student Joel desperately wishes he could become a Rithmatist even though he wasn't chosen. When Rithmatist students start disappearing from Armedius Academy, Joel manages to get himself assigned as an assistant to the professor who is investigating the disappearances, but doesn't realize how dangerous that will be.
Fascinating Illustrations of chalklings and the rithmatists defenses are done by Ben McSweeney.
This is the first book in a series. Go here to read an interview of Brandon about this book.
The author has written both children and adults series. Of those, I've read and enjoyed Alcatraz Versus the Evil Librarians (Scholastic, 2007)--the first of his middle grade series. Doesn't the title just make you want to read it? See more on his website.
Of interest, too, is Sanderson's 3 laws of magic and links to 14 University Lectures by Brandon Sanderson on Writing Science Fiction and Fantasy on this page of writing advice.