I've read a lot of Dean Koontz over the years. There's almost always a dog, generally a Golden Retriever, in his books. At least the best ones. This book is no exception.
I'm not going to give away anything here, I hope. I do encourage you to read the book, however. It's a good story. Good vs Evil with Good triumphant in the end. But there's also a heavy dose of commentary on American culture. That's not nearly so hopeful.
Case in point:
"In the hearts and minds of modern men and women, there is an inescapable awareness that something is wrong with this slice of history they have inherited, that in spite of the towering cities and the mighty armies and the science-fiction technology made real, the moment is fragile, the foundation undermined.
..."But if disaster came, it would be the collapse of civilization, not the end of the world. This blue transparent sky, the sea, the shore, the land, the dark evergreens ever rising--all would endure, unaffected by human misery."
..."Fire, ice, asteroids and pole shifts are bogeymen with which we distract ourselves from the real threat of our time. In an age when everyone invents his own truth, there is no community, only factions. Without community, there can be no consensus to resist the greedy, the envious, the power-mad narcissists who seize control and turn the institutions of civilization into a series of doom machines.
Have a nice day." Relentless, pp.265-266
As depressing as that may be, and the character speaking at that point has every reason to be depressed, there is this redemptive passage at the end:
"Evil itself may be relentless, I will grant you that, but love is relentless, too. Friendship is a relentless force. Family is a relentless force. Faith is a relentless force. The human spirit is relentless, and the human heart outlasts--and can defeat--even the most relentless force of all, which is time." Relentless, p. 356.
Koontz has dealt with some disturbing themes in his novels--from serial killers feed by the theory of utilitarian bioethics to quasi-government agencies bent on ruling the world through the encouragement of chaos and nihilism to voodoo-- and this one is no exception. In the end, I feel he believes in the triumph of the human spirit but not necessarily human civilization. It's a commentary I can relate to in these days of madness.