"Fantastic Voyage" by Isaac Asimov - 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.
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The Rings of Saturn by Isaac Asimov was first published in 1958 as Lucky Starr and the Rings of Saturn, as by Paul French. This is book 6 of the Lucky Starr juvenile science fiction series.
Isaac Asimov's World of Science Fiction : Monsters