Novel about an alternate history where there was no World War II, maybe involving time travel

Elleander Morning by Jerry Yulsman, published in 1984.

A seductive, compelling alternate history in the tradition of The Man in the High Castle.
When the mysterious, beautiful Elleander Morning travels through time to Vienna in 1913, her aim is not to visit the birthplace of Schubert and Strauss. Instead, she has come to assassinate a struggling young artist. His name: Adolf Hitler.

But 60 years on, long after Elleander has changed the path of the world, a mysterious book - the history of a terrible, global war that never was - threatens to unravel reality. As the horrific past - a past that never happened - begins to reassert itself, billions of lives lie in the balance . . .

From a Goodreads review:

In the spring of 1913 an Englishwoman walks into a coffee shop in Vienna and shoots to death a 24-year-old “indigent Austro-Hungarian artist” named Adolf Hitler.

In 1983, that woman’s granddaughter inherits a house in London where she discovers a book entitled “The Time-Life History of the Second World War”, a war she’s never heard of. When she makes the book public, some people are horrified, some are offended - but others are inspired.

Found with a search for science fiction "time travel" "Time-Life History"