Karen Armstrong is a former Catholic nun and studied at Oxford. Her book, The Spiral Staircase, is a good description of the struggles that led to her leaving the convent.
There have been several good books written on the historic Jesus Christ, but very few on the historic God. As other reviewers have noted, this is a somewhat scholarly book, which it would have to be if one wanted to thoughtfully trace back man's evolving beliefs on God. And, yes, over a sweep of 4,000 years, evolving is clearly the correct word.
If you apply the same tools to the study of history of God that one would apply to the study of history of anything over 4,000 years, you will see it through the lens of different periods of time.
Perhaps, somewhat unfortunately for religion and for God, we are in a period marked by the predominance of rationality. Ever since Kant, philosophers have admitted the existence of a god cannot be logically supported (and of course, Kant still willingly chose to believe).
So where does Ms. Armstrong take us in world after Kant, Hume, Schopenhauer, Nietzsche, and the like
She worries about the intolerance inherent in monotheism (if I believe in the one true god, your god must be wrong). She reminds us that although the Existentialists told us we are better off without god since the pat answers and the certainty that god gives stifles our wonder of the world and negates our freedom, the growing drug addiction and crime rates are not signs of a spiritually healthy society.
Apparently although Ms. Armstrong left organized religion, she never left her search for spirituality.
I found the best statement of her conclusion was actually in The Spiral Staircase: "Compassion has been advocated by all the great faiths because it has been found to be the safest and surest means of attaining enlightenment because it dethrones the ego from the center of our lives and puts others there, breaking down the carapace of selfishness that holds us back from the experience of the sacred."
Interestingly, Huxley, who wrote the magnificent Perennial Philosophy on the similarities of mystical experiences across all religions said "It is a bit embarrassing to have been concerned with the human problem all one's life and find at the end that one has no more to offer by way of advice than 'try to be a little kinder." '
A fascinating book that can be recommended to any thoughtful seeker about spiritual matters.