Hollow City by: Ransom Riggs

Wikipedia

In a world full of mystical time loops, blood sucking hollowgasts and flying ymbrynes, one group of kids dare to dream, dreams of freedom, dreams of saving their beloved Mrs. Peregrine. In this second novel of the Mrs. Peregrines home of peculiar children series, the children must embark on a dangerous quest to London in hopes to save the ymbrynes and defeat the wights once and for all. Will they be able to save Mrs. Peregrine before time is up? Will Jacob finally understand his powers? Will anyone survive? (buy it here!)