Starting at the top left and going clockwise:
Winter - Leafless tree from Miss Peregrine's Home for Peculiar Children by Ransom Riggs
Spring - Sun rays (at least from this angle) from The Bone Season by Samantha Shannon
Summer - A rose from The Barefoot Queen by Ildefanso Falcones
Fall - House of Leaves by Mark Z. Danielewsi. Because... you know, leaves.