I now have read five books by Robert Nathan, and this was the third involving a mysterious woman who was much younger than the male protagonist, the others being Mia and the more well-known Portrait of Jennie. As with the other two, the woman never really communicates who she is beyond a first name, and mysteriously comes in and out of the protagonist's life, leading the protagonist to fall in love with her.
The Elixir's version of the formula is the least effective, I think largely due to the narrative being mostly in an uncertain dream world, whereas the previous books had a fairly equal balance of reality and uncertainty. This book may have almost been entirely a fever dream with a framing story of a man on vacation, and when anything is possible it is hard to find something worth grasping on to.