Pastwatch: The Redemption of Christopher Columbus: Orson Scott Card

Book I of Pastwatch.  In the future, a series of machines are created to view past events. Tagiri is a researcher at Pastwatch, the organization in charge of these machines. Her special interest is in the colonisation of the Caribbean and the life of Christopher Columbus. One day she accidentally discovers that the machines may be used to send information backwards in time...the past can be altered...? Columbus had always yearned for greatness but that as a young man he had been determined to head east in a new Crusade to liberate  the Holy Land - so what changed his mind to send him west? Cover art by Thomas Snowdon-Romer.