The City of Three Moons

In 2548 The Council of Ruling Houses of Earth lost the Colonial War of Independence, however, it could still sign new charters with any colony interested in continuing their economic relationship. This is the story of a team sent to reestablish contact and open negotiations with a colony that had been "lost" during the long war.
The planet Eoshia has two moons and occasionally a third that is actually a large asteroid that moves in an elliptical orbit around the sun and then back around Eoshia every five hundred years. The life cycle of the native Eoshians is synchronized with this moon. For part of their life they exist within a global consciousness called Unity. Other times they inhabit a physical body, but only when the third moon is in the sky. As the third moon moves away, the physical bodies die, and their consciousness returns to Unity. The only Eoshians who remain in physical form are guardian/scouts who will prepare for the return of Eoshia's population.
The Eoshian human colony managed to survive during its time of isolation, but had to cope with things such as climate, which was gradually growing warmer, a large poisonous nocturnal animal, called the lamia. Even something as minor as a scratch from the lamia leads to madness and death. To safeguard against the lamia, anyone caught outside after dark is exiled.
The team from Earth includes Dr. Riley Griey, an empathic doctor, and Colonel Mac McKenna. Shortly after they land, they are joined by a representatives from the Proxima Empire, one of the larger and more aggressive of the newly liberated colonies, who want Eoshia for themselves.
Dr. Griey has her own agenda - to find out why the colonists have managed to live many years beyond a normal human lifespan despite the primitive conditions on the planet. She also wants to find the medical empath, Becca, who had been exiled from the colony because of her work with victims of the lamia . When Riley is accidentally caught out after dark, Mac tries to save her, but is himself attacked and badly injured by one of the lamia. He and Riley are banished from the colony and take refuge with Becca.
To find help, McKenna and Riley set out to find a group of scientists who more than 100 years ago, were sent to study the planet. Mac & Riley hope they have will have a cure for the lamia bite, and have answers to many puzzling questions about the planet. However, the Proxima have found the scientists' first and, in an effort to force the scientists to divulge information that will allow them to control the planet, destroy ponds full of large aquatic animals that are actually the young of Eoshia in the beginning stages of metamorphosis. Enraged by this desecration, the Eoshian guardian/scouts and Unity declare war against all humans. McKenna and Riley have just a short time to contact the ship that brought them and arrange to evacuate the colonists. To add to the confusion, however, the Proxima chose this time to attack the main colony and hold them hostage.
After some fighting, bribery, treachery, and so forth, the team from Earth manages to get off planet, taking the colonists with them. The Proxima ignore the warnings and are, we assume, destroyed.