Ghost of the Well of Souls

A soldier and a scientist are trapped in a dimension-hopping war between two alien races, and the story is told through a series of flashbacks and a present-day investigation into their fate — classic Chalker science fiction with a hard-won sense of scale and mystery. The plot is driven by a puzzle (what happened to the protagonists?) and a tense military/espionage backdrop, so military-sci-fi and space-opera fit the genre shelf; the mystery and suspense elements are earned through the layered flashback structure. Moods lean tense and mysterious because the book is built around an unsolved disappearance, with a melancholic undercurrent from the characters' long separation and the war's cost. No romance or horror tropes apply — this is a war-and-mystery-driven space opera.