Wen Spencer, Endless Blue

The Earthlings know that the Nefrim have one objective: to drive them out of space. Thus spaceships have a tendency to vanish without ever coming back; most people assume the hostile Nefrim is behind these assaults. The Nefrim likewise blame the newcomers for their missing ships.

Thus when the vanished Fenrir's warp drive suddenly surfaces at Plymouth Station, no one knows what to make of it as there is no precedent. Captain Mikhail Volkov leads the official inquiry into uncovering what happened from the moment the vessel disappeared during acceleration until the reappearance. He decides the best way to learn the truth is jump to the last coordinates the Fenrir's warp drive displays. He never expected to find himself and his crew in the middle of the oceanic Sargasso universe where no one has ever returned.

This is a superb outer space thriller because Wen Spencer makes the Sargasso sector seem genuine as stranded humans and aliens live in discord. Mikhail is a terrific hero as he and his brother struggle from one disaster into another while beginning to comprehend the real scope of their mission. Failure means the end of at least one universe and probably two.