This is actually two reviews in one. The author, Michael J. Crowe, wrote two books that dealt with the debate on extraterrestrial life. The first book, published in 1986 and the second in 2008, give a comprehensive view of all of the scholarly, religious, and popular writings about extraterrestrials from antiquity (i.e. the Greeks) until about 1915.
His first book, entitled The Extraterrestrial Life Debate, 1750-1900, is more narrative, meaning he does a lot of the explaining and tells the history of the the ideas behind extraterrestrial life. The second book, entitled The Extraterrestrial Life Debate, Antiquity to 1915: A Source Book, contains large portions of the source material that he used to write his first book. Thus it is not narrative, but more informational, with long quotes and brief explanations. Combined both books are a great resource for anyone who wants to know what people thought about extraterrestrials before they became little green men.
I think that if you asked the typical person on the street when was it that we first got the idea that there might be extraterrestrials, they would probably say that the idea of extraterrestrials has only been around for about 100 years, or maybe 150 or so. Very few people would realize that the discussion of extraterrestrials goes back to the Greeks (such as Epicurus and Democritus (yes the guy who thought up atoms, and lost the debate to Aristotle, at least until they found atoms again)). Few people would also realize that the debate about extraterrestrials was also on going through out the middle ages, but that it got much more coverage during, and after, the Enlightenment.
Perhaps the most prominent driver of the extraterrestrial debate was religion, mostly because the question of whether or not there are extraterrestrials directly impacts many religious doctrines and ideas. The author, Dr. Crowe, points this out, but while he does not take a side on the issue, he readily admits that this is a very important question because the discovery of extraterrestrials would radically change the way we view ourselves, our ethics, our culture and religion. One thing that is interesting to note is that while religion has been a major driver in the debate, religion as a whole, or even just Christianity and a whole has not universally taken a side on the debate. Many (almost all) of the theologians in the middle ages took the stance that there were no extraterrestrials, but since the Enlightenment there have been just as many theologians (and even non-theologians) that have taken the view that there were extraterrestrials.
One of the most interesting things I got out of these books was the very different view of extraterrestrials that people had during that time period. Our current view of extraterrestrials has been so radically changed by H. G. Wells, Star Wars, Star Trek, Sputnik, Buck Rogers, E.T., Alien, Roswell and others that it is difficult to get past our current view of extraterrestrials to understand how people viewed extraterrestrials before 1900. Their conception of extraterrestrials was so radically different that most people would not realize just how much our views have changed in 100 years. These books give an excellent insight into how people though of extraterrestrials before there was "The War of the Worlds".
The difference in ideas, and thinking is so profound that any attempt by modern people to understand what was written about extraterrestrials before 1900 will most likely result in misunderstanding, and a severe misconstruing of what people thought and believed. Any attempt to retroactively project our modern ideas of extraterrestrials on what was written before then would be a severe intellectual disservice. The types of questions, dilemmas and motivations in the extraterrestrial life debate are so different from what they currently are that the possibility for misunderstanding and fundamentally missing what was talked about are very high. These books are supposed to bridge the gap between the historical and modern views of extraterrestrials.