I've somehow for years had the idea that Jack Finney's 1970 novel Time and Again (which I hadn't read) was the basis for the 1980 Christopher Reeve movie Somewhere in Time (which I saw some years ago), which I had misremembered as being titled Time and Again.
I recently picked up the novel (having liked some of Finney's short fiction) and was surprised to see that the cover referred to “a top-secret mission,” which I didn't think had been in the movie; but I figured the movie must have been a loose adaptation. I started reading the book, but quickly switched to skimming; it took a hundred pages (a quarter of the book) before the time travel happened. In my head, I congratulated the writers of the movie on having cut most of that introductory material. And then as the plot developed, I congratulated them again on having switched the focus to be entirely on the romance, which seemed to me to fit more with the book's general air of nostalgia than the blackmail plot did. (I wasn't a huge fan of the movie, but I don't think I got as impatient with it as I did with the Finney book.)
So I finished skimming the book, and thought it was odd that the movie was so very different. And I went and looked up Time and Again in Wikipedia, where a footnote says: “Though a film of this novel has never been made, a 1980 film, Somewhere in Time, features a similar time travel technique. It is based on a 1975 Richard Matheson novel called Bid Time Return.”
...Oh. And sure enough, the articles for Bid Time Return and Somewhere in Time confirm that the movie is based on the Matheson novel, and not on the Finney novel. And Wikipedia suggests that the self-hypnosis method of time travel derives from J. B. Priestley's 1964 book Man and Time. (Although possibly in the movie, it's taught to the protagonist by a guy named Finney? I'm a little unclear on that.)
All of which is relatively unremarkable. But what surprises me is that I made such a strong connection between Time and Again and Somewhere in Time without having ever read the Finney novel. I think that long ago I must have read a description of Time and Again that mentioned the self-hypnosis time travel thing, and I must have just assumed it was the basis for the movie, to the point of even misremembering the movie's title.