The first thing to note is that this book does not deserve to be judged as a thesis or a manifesto. Humphrys is a self-professed agnostic, but that does not mean he argues in favour of that position.
Humphrys career as a famous public broadcaster gives him some interesting material for this book: unique interview material from his BBC Radio 4's series "Humphrys in Search of God" with leading religious figures, and a host of letters responding to the broadcasts of these interviews.
Humphrys describes himself as a "failed" atheist, but successfully manages to persuade the reader from early on that he has a keen eye for spurious religious arguments (including those offered by such illustrious people as Rowan Williams, Jonathan Sacks or Tariq Ramadan). The first part of the book is a romp through the case for belief in God, and goes pretty well. The light, almost conversational style serve well - the book is actually a fairly quick read (I read it in one day).
Where he thinks it is appropriate Humphrys shows his dislike of "militant" atheism, and singles out Richard Dawkins for it. Actually, his criticism is well made and deserved. Though Humphrys does not make a meal out of this.
The second part of the book (roughly) deals with belief in god, what it is, how atheists explain it (though Humphrys prefers to consider only naturalistic explanations from evolution, rather than anything from, say, psychology - which is a disappointing limitation to discover).
Finally, although he recognises the dangers of religion in its institutionalised and radical forms, and even though he denies such things as the divinity of Jesus or the authority of scripture, Humphrys does assert three key things that prevent the triumph of atheism:
1. Ethics. It is plain that we have instincts which evolutionists regard as having come from the preservation-of-self and preservation-of-genes instincts, but at times these conflict and what we choose to do is chosen by "something else".
2. Harm. Religion is obviously harmful at times, but also extremely comforting, and it does well to make sense of love that people enjoy, which is quite removed from what evolutionists enjoy talking about.
3. Atheism did not prevent, but was responsible for the greatest evils of history (he mentions Stalin and Mao).
I'll just briefly comment on the last of these points: Stalin and Mao used a perverted quasi-religious ideology for their own ends. Marxism alone does many of the things that religion does: it has its "scriptures", its narratives, its interpretation of history, its moral imperatives, injunctions and prohibitions, its ideologue(s) and its sense of eschatology (the sense of how things will turn out in the end). When Stalin replaced the head of the orthodox church, the Tsar, he effectively replaced the nations tyrant and godhead at the same time. People were used to being oppressed by esoteric doctrines, were used to worshipping a man-god and so on... all of these, and the required credulity, were provided by a prior religious climate.
To return to Humphrys book: there is one thing that really disappoints about it. Its sourcing of information. For the dozens of quotes given, there is not a single citation or reference. We read "Dawkins wrote..." but no book title, year or anything are given. There is no index, no footnotes, no references at the back. This is very lazy. What is even worse though, is that on several occassions he mistakenly gets the name of atheist Sam Harris wrong. He writes it "Sam Smith". Not just once but on several occasions. It's definitely Harris though, since he refers to him specifically as the author of "Letter to a Christian Nation". As though this wasn't bad enough, he does it again in the case of the Polish woman Irena Sendlerowa (aka Sendler), whose name he writes "Sendlerova" (with a v, not a w). Surely Humphrys would know to be careful; his name has been written in innumerable incorrect permutations (Humphries, Humphreys...).
The end of the book summarises Humphrys' reluctance to give up on the idea of God, but he acknowledges that "atheists have the best arguments". So instead he presents God not as an intellectual concept, but an emotional one which is merely useful in getting through life.
But that is a simplistic view which is not sufficiently developed and over-romanticised by far in the book. It raises imporatant questions about the morality of believing something that is false, and that comfort does not confer truth content.
Throughout the book Humphrys says he would like to believe in God, but it is not well explained as to why he wishes this to be the case.
All in all, the first part is very worth reading for some original content and some great debates with major religious figures (e.g. on the problem of evil) and Humphrys gets into full stride with his scepticism. In the second half, we see an attenuated resistance to some nebulous idea of God, which Humphrys is sort of willing to embrace. But we don't know why, or what it is, other than that it has something to do with vague notions of love and beauty.