Rare Earth: Why Complex Life Is Uncommon in the Universe -- by Peter Ward, Donald Brownlee
Makes the argument (IMHO very persuasively) that "life is common but intelligent life is exceedingly uncommon").
Basically their method is to look at all of the 0.0001 probabilities that led to us and throw all those factors at Sagans "billions and billions". Some examples:
--Planet in habitable zone
--Planet with lots of liquid water
--Planet with a large, prograde moon to stabilize its axis
--A gas giant (Jupiter) far enough out not to perturb planet's axis--but close enough to sweep up all the debris which would otherwise destroy nascent life
--Events such as the "Snowball Earth", which filtered 2% of 2% (my estimates) of all life through the event(s), meaning that all life on Earth evolved from .0004 of species previously existing--in only about 600 million years(!)
And so on.
My WAG at present is that there are >1 and <10 intelligent species in our Galaxy--which makes us essentially alone.
Then there is the Fermi Paradox...
--Boris