Strange the Dreamer by Laini Taylor

The dream chooses the dreamer, not the other way around—and Lazlo Strange, war orphan and junior librarian, has always feared that his dream chose poorly. Since he was five years old he’s been obsessed with the mythic lost city of Weep, but it would take someone bolder than he to cross half the world in search of it. Then a stunning opportunity presents itself, in the person of a hero called the Godslayer and a band of legendary warriors, and he has to seize his chance or lose his dream forever.

What happened in Weep two hundred years ago to cut it off from the rest of the world? What exactly did the Godslayer slay that went by the name of god? And what is the mysterious problem he now seeks help in solving?