From Yen Press:
Bound by the promise of a wish granted, magical girls are charged with battling the wraiths that move unseen through the masses of Mitakihara and devour the passions of their human prey. Mami, Kyouko, and Sayaka are of one mind when it comes to fighting these entities, but their task is hindered by squabbles within the group. Mami fears Sayaka and Kyouko will never see eye to eye, even as a giant wraith unlike any they’ve ever fought threatens to make a feast of them all!

An interquel set before Movie 3.