Wow, that title really hooks you—'Once Upon a Time I Loved You' sounds like something I’d pick up on sight. I dug around mentally through the usual suspects—mainstream publishers, indie presses, and library catalogs—and didn’t come away with a single, definitive author attached to that exact English-title novel. Sometimes a book title like this shows up as a poem, a short story, a self-published novella, or even a translated title that varies by region, which makes tracing the original author a little messy.
If you’ve got a copy or a cover image, the quickest route is checking the ISBN or the imprint on the spine; that will point you to the publisher and the credited writer. I’ve had a few late-night treasure hunts in secondhand shops where the ISBN or publisher logo solved what Google couldn’t, so don’t underestimate the cover. It’s a lovely phrase regardless, and whether it’s an indie love-letter or a translated novel, I’d be curious to find the real name behind it — it sounds like the kind of book that sticks with you.