Hollow City by Ransom Riggs (Miss Peregrine’s Peculiar Children #2)

To find a cure for Miss Peregrine, their beloved headmistress, Jacob Portman and his peculiar friends journey through a time loop to London, 1940—the peculiar capital of the world 

Having escaped Miss Peregrine’s island by the skin of their teeth, Jacob and the rest of the peculiar children are fleeing the malevolent  wights following them. Miss Peregrine is the only one who can help, but as she is trapped in bird form, the children must take a time loop to World War II London in search of a cure. 

Along the way, the children encounter new allies and a menagerie of peculiar animals, but hideous surprises and betrayals lurk around every corner. And before Jacob can deliver the peculiar children to safety, he must make an important decision about his love for Emma Bloom.