Four-Day Planet

The eponymous planet, Fenris, has a slow rotation rate, causing days and nights lasting 1000 hours each. This forces the populace to live underground except around sunrise and sunset. And, while this odd planet provides the back-drop to the story, it isn't the story itself. The novel is an example of old-skool sci-fi with competent manly men doing things and not fretting overly much about their damn feelings. It's bracing and refreshing after some of the stuff I've read recently.