I Will Fear No Evil, by Robert Heinlein.The story was first published in 1970, so having read it in the early 80s is very plausible. As to the plot, the Wikipedia page provides a summary (emphasis mine):
The story takes place in the early 21st century against a background of an overpopulated Earth with a violent, dysfunctional society. Elderly billionaire Johann Sebastian Bach Smith is being kept alive through medical support and decides to have his brain transplanted into a new body. He advertises an offer of a million dollars for the donation of a body from a brain-dead patient. Smith omits to place any restriction on the sex of the donor, so when his beautiful young female secretary, Eunice Branca, is murdered, her body is used. He changes his name to Joanne Eunice Smith.
After Smith awakens after the transplant, he discovers he can communicate with Eunice's personality. They agree not to reveal her existence, fearing that they would be judged insane and locked up. Smith's identity is unsuccessfully challenged by his descendants, who hope to inherit his fortune. Smith and Eunice decide to have a baby together and so they (Joanne) are artificially inseminated using Smith's sperm from the sperm bank. Joanne (Smith) explores her new sexuality at length. They (Johann & Eunice/Joanne) go to visit Eunice's widower, Joe Branca, to try and help reconcile him to what has happened.
Joanne marries her lawyer, Jake Salomon, and moves her household and friends onto a boat. Jake has a massive rupture of a large blood vessel in his brain and dies but his personality is saved and joins Smith and Eunice in Joanne's head. She (Smith, Eunice & Jake) emigrates to the moon to find a better future for her (Smith & Eunice's) child. Once there, her body starts to reject her (Smith's) transplanted brain. She dies during childbirth.
The chant mentioned in the question sounds very much like "Oṃ maṇi padme hūṃ", a traditional mantra in certain sects of Tibetan Buddhism. With a nod to Ross Presser for tracking this down, here's a quote from the book demonstrating its use:
“Start us. Remember the breathing. I’ll get in step.”
“Om Mani Padme Hum.”
(Om Mani Padme Hum. See that aura round her, Boss? She must have
had quite a night.) (Shut up, Eunice; these prayers were your idea.) “Om Mani Padme Hum.”