From the Sea to the Stars: Sea Siege Star Gate by Andre Norton (2011) Hardcover

From Grand Master Andre Norton comes From the Sea to the Stars-two of her earliest SF adventures, together for the first time in one hardcover volume. Sea Siege: The research team on a Caribbean island think they're lucky to have survived the nuclear holocaust. But they'll need more than luck when a horror more terrifying than the war awakens from the sea-determined to destroy what's left of the human race! Star Gate: Long ago, the Star Lords settled on planet Gorth and helped its primitive inhabitants rise to civilization. Now some of the Lords have found a parallel Gorth, but in this universe, they conquered and enslaved the people. To free them, the Star Lords must battle their counterparts-even if it means their own destruction.