Total Recall by Piers Anthony

ReKall - the multimillion manufacturer of synthetic memories. In the latter part of the 21st century their trained mind-implanters can make the impossible possible, adding spice to their clients' mundane existences. But construction worker Douglas Quaid's vividly imagined Martian fantasy masks a deadly secret: his true identity. And he's being haunted by a killer from an invented past that has suddenly become frighteningly, inexplicably real.  Avon Books 1990 paperback in good condition, see photos.