It may not be a bad book per se, I'm just having hard time figuring out why it was written. A typical chapter opens with a brief story of a massive killing or destruction that took place in the history of the mankind (for example, Sodom and Gomorrah), then comes an overview of a source (most often it's Zecharia Sitchin or the TV series "Ancient Aliens") saying that this destruction could have been a result of extra-terrestrial intervention, followed by the part where the author ponders on whether it was really the case. So far so good. The annoying part is that Mr. Redfern never really tells us what his personal opinion is on the matter, so even if it is implied that he is inclined to debunk the ET hypothesis, he still ends the chapter with something like a non-committing "but who knows, right?".
Some chapters are better than others. For example, I found the one dedicated to the 10 plagues of Egypt bordering on the ridiculous, but the one on the city of Mohenjo Daro was okay. Overall, the narration is very uneven. At some point you have a feeling that here, the author is finally going to come to some exciting or definitive conclusion, but--alas--it never happens.
I think the notion of the book is best expressed by the author himself in the Conclusion: "We have to look at both sides and recognize that, whether we like it or not, there are two sides of the story. .<...> We need to highlight not just the supportive material, but the undeniably flawed data, too. Only by doing so can we determine if atomic weapons, built and deployed by alien visitors, were used on our planet long ago. Or were not." This is fine, albeit commonplace. Still, even using this balanced approach principle, the author achieves nothing in determining anything.