The speaker imagines a relationship as a closed book, with a ray of sunlight tucked between the pages like a bookmark. The sun "marking our place" suggests the couple has paused rather than ended. A little warmth, a "ray of memory," still sits inside, holding the spot where they left off.
The second half turns the book into two bodies, or two lives, pressed together. The "hills and dales" of the "embossed covers" feel like landscape and like the raised texture of a bound book at once. These covers "sipped essences from each other," so even while closed, the two have been quietly exchanging something of themselves. The last word, "foment," is left hanging. It means to stir up or incite, so the closeness seems to be building something, perhaps desire, perhaps unrest.
The mood is hushed and intimate, tinged with nostalgia. It feels private, tender, and a little charged, as if the story isn't finished.