Carl Sagan : A Life in the Cosmos by William Poundstone; (see also my review at Carl Sagan : A Life by Keay Davidson - this review considers both books)
Carl Sagan is easily the second most famous scientist of the 20th century. If you came of age in the period 1970-1990, you were influenced by Sagan - period. Whatever you may think of him as a scientist, you must admit that nobody did more to popularize science in the public eye during this period. The two most obvious examples are his Cosmos television series and his numerous appearances with Johnny Carson on the Tonight Show.
Poundstone's book reflects Ann Druyan's influence much more than Davidson's. The result is a much more flattering account of Sagan's life, potentially minimizing some of the warts. Davidson, if anything, spends too much effort trying to psychohistorically analyze Sagan's two failed marriages and his fractured relationship with oldest son Dorion.
Davidson also focuses much more attention on Sagan's books, attempting to plot the development of his career as a scientist and maturity as a writer based on each book's unique character. Here again, he attempts to delve below the surface into the hidden motives and influences. For instance, while both Poundstone and Davidson detail Sagan's marijuana use, Davidson goes further and suggests that the Pulitzer-winning Dragon's of Eden was largely a marijuana- induced work.
William Poundstone Focuses more on his scientific achievements, with emphasis on the many conferences he chaired regarding SETI, exobiology, and his work on the Voyager and Mariner probes to Mars and the gas giants. Some of the reviews of the latter actually read like a popular scientific account of these missions, written around Sagan's contribution and perspective.
A very rough generalization would be that Davidson looks more closely at Sagan's personal life while Poundstone looks more closely at his scientific achievemnts, though both books do cover the whole picture. Poundstone's book left me with more of a positive regard for Sagan though, and struck me as the better book of the two. Poundstone's account strikes me as first and foremost a work of scientific biography, with more detail of Sagan's scientific achievements.