Extraterrestrial beings are a standard part of science fiction, but in the real world there doesn't seem to be much evidence for them. In Why aren't they here?:The Question of Life on Other Worlds
Surendra Verma considers why this should be. His starts with ideas about extraterrestrial life in history, starting with Aristotle, and goes on to look at early examples of extraterrestrials in science fiction. There is then a chapter on the origin of life and the possibility of panspermia followed by a look at what planet environment is needed for life and whether there are any likely candidates.
Verma considers the various hypotheses which people have put forward concerning the probability of extraterrestrial beings, and the reasons why these beings haven't contacted us yet. He goes on to describe the various projects which listen for signals from alien civilisations, and considers what we might do for a reply. The last chapter gives short answers to various other questions about extraterrestrials. I have to say that I felt that some of these questions really deserved more space. Verma has written several popular science books, and in this one he takes every opportunity to explain the science behind what he is saying. So possibly this book isn't for those seeking detailed discussion about extraterrestrials, rather it's a fast paced but easy to read book which bridges the gap between science fiction and science fact.