What the genre of a book call 'space'?

The title of a book isn't enough to determine its genre.
It could be about anything.

Space opera is a subgenre of science fiction that emphasizes space warfare, melodramatic adventure, set mainly or entirely in outer space, and often risk-taking as well as chivalric romance; usually involving conflict between opponents possessing advanced abilities, futuristic weapons and other sophisticated technology. The term has no relation to music but is instead a play on the term "horse opera", which was coined during the heyday of silent movies to indicate clichéd and formulaic western movies. Space operas emerged in the 1930s and they continue to be produced in literature, film, comics and video games.

Notable space opera books include the Foundation series (1942–1999) by Isaac Asimov et al. and the Ender's Game series (1985–present) by Orson Scott Card. An early notable space opera film was Flash Gordon (1936–present) created by Alex Raymond. In the late 1970s, the Star Wars franchise (1977–present) created by George Lucas brought a great deal of attention to the genre.