Poetic sci-fi forbidden romance

How to describe this book? It's sci-fi, it's a romance, it's sapphic, it rightful went viral on #BookTwitter, it's told through letters. I think it's easiest to say that the plot is less important than the emotions, and this book made me feel so many things! While I couldn't always visualize what was happening, I didn't really mind because the writing was so beautiful. The world building was set aside in favor of focusing on the character development. This wasn't what I expected when I picked the book up, but I think it paid off. Other books that I've read told through letters can leave characters two dimensional, and I struggled to care about those characters.
This Is How You Lose the Time War presented the opposite - I didn't mind what happened in the plot, I was so invested in how the characters were feeling.

An eternal romance I could believe in. Loving, tender, and ever surprising. Such a beautiful ending.