Fantastic Voyage II: Destination Brain is a 1987 science fiction novel by Isaac Asimov about a group of scientists that shrink to microscopic size in order to enter a human brain so that they can retrieve memories from a comatose colleague.
Four men and a woman are reduced to a microscopic fraction of their original size, sent in a miniaturized atomic sub through a dying man's carotid artery to destroy a blood clot in his brain. If they fail, the entire world will be doomed.
Extraterrestrial Civilizations is a book written by Isaac Asimov in 1979, wherein the probability of there being intelligent extraterrestrial civilizations within the Milky Way galaxy is estimated. This estimation is approached by progressively analyzing the requirements for life to exist.