This a long book (almost 800 pages) so it took me a while to finish.  No spoilers.  There is a group of people called walker who suddenly leave their homes and start walking.  In the beginning of the book you think it is caused by one thing but as you go through the book that changes a couple of times.  You are discovering the nature of the phenomenon along with another character.  There is a substory of a global pandemic so if that is too much for you right now skip reading this book.  The ending is also not something I had expected but was definitely hinted at throughout the last half.  I enjoyed the story and development of the characters.  Recommend.