The Time Traders: A Science Fiction, Post-1930, Adventure, Espionage Classic By Andre Norton! AAA+++

Intelligence agents have uncovered something beyond belief, but the evidence is incontrovertible: the USA’s greatest adversary is sending its own agents back through time! And someone (or something) is presenting them with technologies and weapons far beyond our most advanced science. We have only one option: create time-transfer technology ourselves, find the opposition's ancient source...and take it down!