S. M. Stirling - Nantucket Series (#1-3)

Island in the Sea of Time (Nantucket #1)
During a perfect spring evening on Nantucket a violent storm erupts and a dome of crawling coloured fire blankets the island. When the howling winds subside and the night skies clear, the stars appear to have shifted. The mainland has become a wilderness of unbroken forest, where tools of bronze and stone litter the beaches, and primitive natives scatter in terror.

A startling phenomenon has occurred; The island of Nantucket has been swept into the long-ago past. With its inhabitants adrift in the year 1250 B.C., there is only one question to be answered: Can they survive?

Against the Tide of Years (Nantucket #2)
Continues the adventures of the Nantucket residents who have been transported through time to the Bronze Age. In the years since their arrival, the fledging Republic of Nantucket has striven to better the primitive world in which they now exist. Their prime concerns are establishing a constitution and handling the waves of immigrants from the British Isles. But a renegade time traveller plans his own future by forging an empire for himself based on conquest by modern technology. The Republic has no alternative but to face the inevitable war brought on by one of their own.

On the Oceans of Eternity (Nantucket #3)
Ten years ago, the twentieth century and the Bronze Age were tossed together by a mysterious Event. In the decade since, the Republic of Nantucket has worked hard to create a new future for itself, using the technological know-how retained from modern times to explore and improve conditions for the inhabitants of the past. Some of these peoples have become allies. Some have turned instead to the renegade Coast Guard officer William Walker. And for ten years, the two sides have tested each other, feinting and parrying, to decide who will be the ones to lead this brave new world into the future. The final battle lines have now been drawn. And only one side can emerge the victor.