You might be surprised that the universe's ability to support life may have been affected by the way that stars produce carbon in just the right quantities. Carbon atoms are produced by combining the particles from the nuclei of three helium atoms. This is done in the nuclear furnaces of the stars, but therein lies a problem. Two helium-4 nuclei might collide and fuse to create a beryllium-8 nucleus quite often but it is very unlikely indeed that three heliums will collide at exactly the same moment to make carbon-12 in one step. In fact the process goes in two steps, with the second step being the fusion of beryllium and helium ions to make carbon.
The problem is that beryllium-8 is VERY unstable, (lasting for only for a thousand million millionth of a second), so it is surprising that it lasts long enough to combine with another helium. The fine tuners claim that the existence of a particular excited state of carbon makes the reaction very much easier because of a phenomenon known as a resonance, and physicists agree. However, they say that the energy of this resonance has to be exactly the value that we find, or else it will not work. There is no need to understand the details because Stenger explains that it is not true to claim that the numbers have to be so accurately defined. Going through detailed mathematical explanations he shows that the energy of the excited carbon could have been anywhere in quite a wide range - including the value that is observed.
One of the interesting aspects of the whole saga is that carbon combines further with helium, and that in doing so it creates oxygen-16, which is also critical for life of the form that we know. It is just as well that this reaction doesn't happen too easily or there would be no carbon left. But this brings us to another point.
Who can say that in an alternate universe with different physical constants it might not be possible for life to be based on a different chemistry. The claim that it can only be based on the chemistry that we have in our universe is sometimes known as carbon chauvinism! Life might flourish with a different form of tuning.
I should also say that Stenger took care not to rely on the concept of a multiverse in order to refute fine tuning arguments. He managed it quite well enough by using evidence from the one universe that we know about.
I'm not going to cover any of the other interesting aspects of the book in any detail here, but he mentions the poor arguments using probability, and spends a lot of time on the first instants of the universe. Fine tuners often take a sample sentence from Steven Hawking's book, A Brief History of Time out of context, rather than reading the full explanation about how it would lead inevitably to much of the claimed fine tuning of the universe's expansion rate.
Stenger also mentions the good reasons for some of the other fine tuning claims, such as why we should not expect there to be different numbers of protons and electrons (to a very high accuracy).
In conclusion I would say that he covered the subject very thoroughly, sometimes using more mathematics than I wanted to see, but that it is easy to get a lot out of the book without a full understanding of the maths.
As he points out, in order to refute claims of fine tuning arguments he does not actually have to give any reasons for his claims, but only to show that a wide range of parameters could lead to a universe that supports life.
I think he succeeds!
Stenger 1 : God 0!