The foundation of this Hugo and Nebula Award winner's latest series is an interstellar war with the mysterious Fallers, a civilization so alien there has never been any communication other than killing. Both sides use a little-understood series of space tunnels left by a vanished race.
The first in the series, "Probability Moon" introduced World, a planet of empaths whose "shared reality" makes lying impossible. While a team of anthropologists established relations with the Worlders, a military team studied the planet's artificial moon, another of the vanished race's artifacts, which they hoped would turn the tide in the war. The story ended in disaster, with the humans declared "unreal" and the moon destroyed.
The sequel, "Probability Sun," neatly telescopes the earlier story as humans prepare for a new mission to study a second artifact hidden in World's sacred caves. The mission includes two characters from the first book, blunt, straightforward geologist Dieter Gruber and his thoughtful wife, xenobiologist Ann Sikorski as well as brilliant, eccentric physicist Tom Capelo, gene-engineered empath Marbet Grant and Major Lyle Kaufman, the mission's reluctant leader, a mild, politic man who doesn't recognize his own strengths.
While the scientists swarm over the artifact and re-establish relations with (and studies of) the Worlders, including Enli, whose previous experience gives her more insight into humans than she wants, the military secretly uses Marbet Grant to study the first Faller ever captured alive.
The character-driven action moves between the ship and the planet, the alien enemy and the enigmatic artifact, military ambitions and scientific goals, building to choices that may destroy Worlder civilization, tip the balance of the war or end the universe as we know it. Kress' story is well organized and well written and her characters multi-dimensional. The story is an engaging blend of military and psychological strategy, speculative science, moral dilemmas and suspense. The ending provides satisfying closure while leaving the door open for a third book.