"The Sky is Falling" is a 1973 science-fiction/fantasy novel by Lester Del Rey, published as an Ace Double, and expanded from a short story published in 1954. The other side of the Ace Double is "Badge of Infamy", also by Lester Del Ray. Being published in 1973, this was one of the last Ace Doubles published. Also, interestingly enough given Lester Del Rey's long career in science-fiction, this was the only Ace Double he appeared in.
The story begins, in media res, with Dave Hanson waking up in a hospital bed. But this isn't a normal hospital---the doctors, such as they are, are discussing his astrological sign and the conjunction of the planets. It turns out that Dave Hanson died on earth, and was summoned to another planet/dimension, because, as the title states, the sky is falling. In this planet, magic is real and the sky is a dome of glass like material, and Dave Hanson, a computer engineer on earth, is seen as the one person who can fix it---because they are confusing him with his uncle, also with the same name, who is a brilliant engineer. But Dave Hanson also has to solve the mystery, even while being kidnapped and magically transported around the dying world. In the end, after accepting the existence of magic on the world, he is able to use his own knowledge of computing to create a device to save the world.
Ace Double books are often full of sudden changes of scenery, and characters change allegiances and location with rapid frequency in a 100 page book. Ace Doubles also often mix science and fantasy elements. One of the strengths of this book is that it deals with all of that internally, and the protagonist is surprised at the crazy patchwork nature of the magical world. So when a chapter of this book suddenly has Dave Hanson working building an Egyptian pyramid, it makes more sense than in a book that tries to be hard science. The book has a few philosophical points (especially about the nature of reality, and a moral dilemma about saving a static and unequal society from disaster), but in general, the author uses the surreal setting of the magic realm to tell an adventure story.