The Seaborne, Book 1 of The Isle Fincara Trilogy; A. G. Rivett.

A tale of metamorphosis in which two worlds collide and John, a 21st century Londoner, is compelled back in time to the Island, where he must learn to live anew. A modern engineer. A medieval setting. A clash of cultures. A Celtic fantasy. And a love story, beautifully told.