I found this book when one day it sank in that I could search ABE/Bibliofind by publisher, so I put in the Hokuseido Press, one of my favorites. I found a number of interesting titles this way, and by cross referencing on AltaVista, I came across this site about Izumi Kyoka.
In this, the HTML author mentions "a heartbreaking little book called Ten no Yugao (A Moonflower in Heaven) by Yoichi Nakagawa", which I decided to order.
The book's introduction compares it to Goethe's Werther, which worried me, as I read that book when I was young and thought it pretty silly. And in fact, this love story is also very simple and old-fashioned. The man and woman love each other, but she is married, and though her husband is traveling abroad all the time, they can do nothing. Twenty-five years or so pass, during which they are mostly thinking of each other, and she dies when he is 43, a few days before they were going to get together. He is heartbroken, but shoots off fireworks: flowers for her to pluck in heaven. A sweet image.