John Brunner 
Threshold of Eternity [EPUB ebook] 

Because of a twist in the structure of Time, three strangers were brought unexpectedly together: Red Hawkins of California, Chantal Vareze of London and a man from the 41st Century. Their meeting seemed an impossible prank of a universe gone mad — but it turned out to be quite otherwise. For it seemed there was a war going on throughout space and time. A war fought by men of different epochs, on planets of different cultures, but for a cause that all could acknowledge — the very continued existence of creation itself. And the coming together of these three very unlikely people — a modern man, a lovely girl and a futurian soldier — was to prove the master stroke of a super-science strategy that had already brought humanity to the THRESHOLD OF ETERNITY.