Eli Bendersky's website The secondary title of this book is: "Science as a candle in the dark", and both titles hint quite accurately what the book is about.
Carl Sagan takes on explaining in what way science is good for humanity, and how pseudoscience (including "alien abductions", mediums, astrology, ghosts, demons, spoon bending, crop circles, revelations of gods/angels, wonder therapy and so on) is bad. The first half of the book pounds pseudoscience and the second one is more political and philosophical, debating the place science holds in the current world - in education, economy, and so on.
When Sagan opposes "aliens" it's not that he doesn't believe other life forms exist (quite the contrary, as people who read his other book know). He fights the "flying saucer" image portrayed by people who want attention and promoted by tabloids. All in all, IMHO he spends a little too much time on the subject, it becomes quite tiring. Maybe it's because the evidence of aliens is presented in such ways as to be deliberately difficult to disprove. On the other hand, crop circles and wonder medicine are easier to disprove, and Sagan presents some interesting facts and researches on the subjects.
A "Baloney detection kit" is presented where the reader is encouraged to learn how to distinguish solid scientific material from "baloney" pseudo-science. The role of critical thinking in science is emphasized throughout the book.
Most of the second half of the book deals with how bad US schools are in science, what unpleasant implications in may/will eventually cause, and what the goverment can do about it. A little bit too political to my taste...
I must say that of all Sagan's books I've read so far (quite a few), this one I liked the least. Maybe it's because he deviated from his good'n'old scheme of popularizing science itself.