Guardians of the Herald: Angels and Demons - Guardians of the Herald, #1

Billy Ransom has been recruited by a small band of civilian and military personnel who use technological advancements to send agents, called Guardians, into an ethereal world overlapping our own. On an anti-terrorist mission the Guardians discover they are not the only ones venturing into the ether when they encounter a battle between an angel and a demon over possession of a human soul. What happens next draws the members of the Guardians into a much larger conflict, bigger than any of them imagined, a conflict in which hangs the fate of every human soul to have ever existed.