Something recently reminded me of a science fiction novel I read back around the mid-1980s (say, somewhere in the range of 1983-1986). It was written in English, published in hardback, and I checked it out from a public library in Indiana. I don't think I recognized the author's name from anything I had read before.
I only remember a few bits and pieces of the plot. I suspect I'm forgetting more than half of the conflicts which arose among the major characters. But here's what I've got.
The basic premise was that it's sometime in the future -- but I don't think the human race had expanded outside this solar system -- and it has become not only possible but even commonplace to somehow download a copy of a dead man's memories and personality into the head of a living, breathing man. (I said "man" because the main instance I remember was male, but I expect the process worked equally well for women.)
In theory, the original personality was supposed to stay dominant, and the implanted personality was supposed to just whisper helpful suggestions (as silent thoughts which no one else would hear). Business advice, for instance. Or perhaps technical skills. I don't remember exactly how it was rationalized as a great idea, but a great many people had volunteered for such procedures. In theory, it was very useful in making you more effective in handling a variety of problems.
In practice, it sometimes happened that the implanted personality somehow managed to seize control, briefly or as a long-term thing, of the body, and suppress the personality of the original owner. This, of course, was treated as a major felony if it was defeated. I think it was possible for the experts to reverse the process by somehow removing the implanted personality from the brain. I don't remember why anyone thought it was such a brilliant idea to volunteer for this procedure if there was a significant risk of becoming a helpless prisoner inside your own skull.
There was an odd word which was used for that situation of "the implanted personality has seized control and doesn't want to let go." I think the word may have been "dybbuk" -- a word which I didn't recognize at the time, but ran across again, years later, in other books. (It turned out that "dybbuk" is a Yiddish word referring to a concept in Jewish folklore -- the spirit of a dead person which is maliciously trying to possess a living, breathing person. That would be a very good fit for the way I think this word was used in the novel, so I suspect the writer did, in fact, use it to suit the occasion -- but I'm not sure he didn't use a different odd word.)
One of the major characters in the book is having problems with his implanted personality, of course. I don't recall his name, but let's call him Flunky. I say "Flunky" because I believe he was a faithful employee of another important character; some sort of rich and powerful business executive who may have paid for Flunky's personality implant in the first place. The idea seems to have been to make Flunky, who was a rather passive personality, into someone more clever and aggressive.
We soon learn that Flunky has so much trouble with his implanted personality (hereafter dubbed Dead Guy) that Flunky has adopted the habit of constantly carrying on his person a small bottle containing a lethal dose of some fast-acting poison. The idea is that if he ever feels Dead Guy is on the verge of seizing control of Flunky's body, then Flunky will whip out the bottle and take a hefty slug of poison. He hopes the knowledge of this possibility will deter Dead Guy from trying to stage an internal hostile takeover.
There were other complications in the plot, but they have completely slipped my mind. All I remember is that, for some chapters, Dead Guy was, in fact, in the driver's seat in Flunky's body. Dead Guy even rekindled an affair with a woman who'd been his lover until Dead Guy's original body died some years earlier. (I believe she knew Dead Guy was back, and preferred it that way, so she wouldn't dream of telling the cops they had a "dybbuk" situation.
At a climactic moment near the end of the book, something happens -- don't ask me what! -- which shakes Dead Guy's grip and allows Flunky to seize control of his own destiny again. Flunky makes a brief speech, assuring his rich boss (and anyone else in the room) that he had, in fact, been "dybbuk," but right now he isn't, and he's going to make sure he never will be again. Then he whips out the aforementioned bottle of poison (which Dead Guy had never bothered to get rid of during the days he was in control) and kills himself, thereby taking Dead Guy with him.
At the very end of the book, I think it was either stated or strongly hinted that the rich boss's Beautiful Daughter had secretly had some other controversial person's memories copied into her brain. (I don't recall if it was another family member, or one of her father's biggest rivals, or what.)
I don't remember a thing about the author's name, nor the character names, nor even the cover art. I strongly suspect it was an author who is now fairly obscure, since I don't recall running across this book again -- or even a reference to it -- within the last couple of decades. (That tends to eliminate such big names as Poul Anderson, Samuel R. Delany, Philip Jose Farmer, Harry Harrison, Robert Silverberg, etc.)
So, does anyone think this sounds familiar? (Bearing in mind that it was published no later than 1986, and I suspect much earlier.)