Of Beginnings and Endings (Castaways in Time, No. 6)

Of Beginnings and Endings (Castaways in Time, No. 6) by Robert Adams. mass_market edition. ISBN: 9780451159724.

Thrown back through the centuries to a war-torn Britain and America, Bass Foster and his fellow castaways control kings and countries by wielding weapons from a highly advanced future civilization. But the unsuspecting time travelers have attracted the attention of mysterious, awesomely powerful beings who might bring the world tremendous benefit - or destroy it with terrifying force.