This is part of a series of Ace science fiction books, each containing two complete novels, both with front covers; so half the book seems to be upside down if you are just flipping through the pages. Robert Silverberg wrote Master of Life and Death, and James White wrote The Secret Visitors. Silverberg's story deals with overpopulation, the search for immortality, and politics. White's story has aliens from outer space coming to Earth and an interesting alien species.