I recommend reading this essay by Nick Bostrom at MIT's Technolgy Review. It is much faster than reading a book by Carl Sagan or Fred Hoyle on extraterrestrial life or any of these . This essay brings home strongly the most important part of the questions of where the aliens are to be found: does their absence hint that we too must inevitably parish before we can carve our initials on far flung suns and call out to the universe saying "Here we are, where are you?". Don't ever let the believers think they have cornered the market on the most teleological of concerns...they are pikers beside a good astrophysicist.
Not that I expect much comment but if you are inclined to question some of Bostroms leaps and assumptions make sure you have properly considered how fleeting our species rise has been in the scale of all the existence of the universe...some 13 billion years at least. Assuming we are nothing special means there has been more than enough time for the half of galactic life that came before us to have evolved and conquered all the distances we can see. There has definitely been time. And the lack of evidence of such conquests means there never will be any.