The book details a number of the parameters, forces, constants and ratios that have to be just what they are to a breath-takingly fine precision or else the universe either wouldn't exist or wouldn't be the sort of place that could sustain life.
I thought the book to be so important and the style in which Barnes and Lewis compose it to be so accessible to laymen that when I came across this short video publicizing it I thought it'd be good to post it on VP in hopes that some readers may want to read the book.
This cosmic fine-tuning as it's called constitutes a powerful cumulative argument for the existence of an intelligent mind responsible for it all. There seem to be no other very plausible explanations, but some who are queasy about the support fine-tuning gives to traditional theism have adduced other possibilities. Some have posited that our universe is the product of a computer simulation somewhat like the Matrix. Of course, this explanation still relies on an intelligent transcendent being. Others have sought to abandon the idea of an intelligent creator altogether and have embraced the idea of a multiverse which incorporates every possible universe in one unimaginably vast array of worlds. If such a multiverse exists, the thinking goes, then since our universe is possible it must exist somewhere in this enormous ensemble.
So, there are essentially three possible explanations for why our universe exists: A computer simulation designed by a mind in some other world, an infinity of universes (Geraint Lewis' position), or theism (Luke Barnes' position). The problem is that both of the first two explanations themselves must be explained. If the creator of our world is an alien computer wizard, then how did the wizard come to be? Or, if the reason for our universe is some sort of multiverse generator, how did that come to be?
On the other hand, if the creator of the universe is the God of classical theism then the creator is a necessarily existent mind upon which all contingent existents depend. The creator's existence requires no further explanation because the creator is not a contingent being. The explanation of its existence is in itself. Here's a video I posted with the original piece on A Fortunate Universe which elaborates on this concept: