Walls. The Nexus Chronicles, #1

The Nexus. Few know of it, and fewer know what it really is. One of those people is Alastair MacMillan and he knows more than most. This information has left him with one world-altering choice. When the Nexus opened, rifts tore through the boundaries. Earth became home to creatures that could only be described as impossible, both sentient and not. They found they could not return to their home realms.
From walking plants and living shadows to being of psychic energy and pure elemental states, the four other realms intersecting Earth bring horror. These creatures invaded Earth, sending the population scrambling to protect themselves. Many lost their lives, but those who remained built walled cities and other tools to keep the creatures at bay. Allies and enemies are made among the sentient species, some of which decide to assist humans in defending themselves.
So Alastair has to make a choice. His father told him before he died that he had to choose what to do with the knowledge he had given him. He could either use it and maybe save the world, or he could ignore it and let the world die a gasping death. When he finds a reason to live, though, the choice may be easier than he imagined.