Trying to recall a novel, read in late 80s/early 90s but probably from the 60s/70s.
The protagonist was investigating cases of spontaneous human combustion, which turned out to be due to aliens (living in an asteroid?) swapping bodies/minds with humans. When the sun was in the way/interfered in some way, the humans burned to death. Otherwise, their consciousness was swapped into the alien body.
I believe this is Fire Pattern by Bob Shaw.
Published in 1984 and set in 1996, Fire Pattern has the words "science fiction" on the back cover but starts out much like a conventional thriller, one of those books about an ordinary middle-class person facing quotidian life challenges who starts a new love relationship while doing detective stuff and uncovering a conspiracy. Fifty-year-old Ray Jerome is still recovering from the death of his wife and the loss of his job as an engineer. His current job is as a reporter at a small New England newspaper (British subject Shaw set this novel in the United States, apparently an alternate universe USA in which Americans measure distances in "kilometres" instead of "miles" and call their summer homes "chalets.") Jerome, who is older than everybody in the office and worked at a trade journal, is always correcting everybody's grammar and writing style and ridiculing people's beliefs in goofy nonsense like spontaneous human combustion. Circumstances force him to change his tune about fortean phenomena when a case of "SHC" occurs in his little New England town, and the editor of the paper (a forty-year-old woman our hero has a crush on) assigns Jerome to investigate.
Like in any detective story, Jerome talks to the witness, reads background material, visits the morgue, looks for clues in photos, blah blah blah. You can bet I was relieved when, 75 or 80 pages into the 208-page paperback, we plunge into Van Vogt territory! Cases of SHC are revealed to Jerome to be botched mind transfer operations originating from Mercury! For centuries the telepathic Mercurians (who call themselves "Dorrinians"), by the hundreds, have been colonizing Earth by switching bodies with Earth people (and in the process building up a colony on Mercury of Earthlings in Mercurian bodies!) Jerome finds himself in the middle of a secret war between factions of telepathic Mercurians on Earth, and then, in classic wish-fulfillment fiction style, is transferred to a young healthy body on Mercury! In the first part of the book Shaw kept reminding us of Jerome's arthritis and poor eyesight, making the move to the Mercury body cathartic. Even better, the Terran colony on Mercury has a culture of sexual promiscuity--I mean free love!
I found it via a search for science fiction novel alien possession "spontaneous human combustion". I was lucky enough that it showed up on the first page of results.
