"Flight from Yesterday" is a 1963 science-fiction novel by Robert Moore Williams, published as an Ace Double along with Envoy to New Worlds. Williams was a very prolific author of short stories, and had been active in science-fiction for over two decades when this was published, but despite his large output, he never gained much fame in the genre.
The book takes place in a (for the time) near-future setting, the world of 1980 Los Angeles, California. Keth Ard is an out-of-work test pilot who answers a help wanted ad and goes to a seedy pawnshop where he sees a beautiful redhead and then the owner of the pawnshop tries to kill them with some type of scorching death ray and they must both flee to the home of his psychiatrist, a kindly man who treats them and where they discover that they are both mentally connected to a pair of lovers in doomed, ancient Atlantis and that the pawn shop owner is being possessed by an evil wizard from Ancient Atlantic, and in the present, they must keep a magical amulet out of his grasp and then, while Atlantic trembles under a giant earthquake, in modern LA they have to fight off the evil wizard/pawnshop owner and...
Well, I don't want to give it all away.
There are a lot of ideas here. Too many ideas are kind of the bane of Ace Doubles, unless they are written by Michael Moorcock, in which case that is to be expected. As is often the case, I blame the editors at Ace. The standard way this book could have been written would have been to begin in LA, flashback to Atlantis for the bulk of the book, before reaching our conclusion. The book talks about Atlantis---but mostly takes place in a noir environment in LA with only a tenuous connection between the present and the past. It is a somewhat odd construction---but I kept reading.
Another thing to note about this book is that while our protagonist is a tough guy test pilot, the good characters adhere to non-violence as much as possible, and are reluctant to use weapons. The greatest good in the books is shown in the medical devotion of Keth Ard's psychiatrist, and in the gentle teachings of a good wizard from ancient Atlantis. As is often the case, pulp science-fiction was much less "two-fisted" than it is believed to be.