Frozen (Heart of Dread #1)

By Melissa de la Cruz and Michael Johnston
Putnam Juvenile

Set in 111 C.D., one hundred and eleven years after a Catastrophic Disaster has wiped out 99% of humanity and left the earth covered in ice, this new series introduces readers to a ragtag group of friends and the dawning of a new time. The world of reason, of mathematics and science, is ending, and a new civilization is being born from the ice: a world of magic and mayhem, sorcerers and spellcraft.

(Image and summary via Goodreads)