Midday Moon

Amano is a human who works at a small, strange inn on the edge of civilization that serves as a refuge for the Others--immortal visitors from the everlasting world, which exists parallel to the transient world of mankind. When Tsukuyomi, the king of the world of the deathless, pays a visit to the inn, it's up to Amano to tend to his every need. But to his shock, the haughty, beautiful king is the spitting image of his old childhood friend who disappeared under mysterious circumstances long ago! Tsukuyomi's world may be far, far removed from Amano's own...but in a moment of passion, Amano vows to always stay by the king's side.

(Source: Bookwalker)

Note: Polar Night is a reprint of Mangetsu no Ou, but with corrections, 36 additional pages, and the inclusion of doujinshi the mangaka wrote after the main story was first published. However, it does not contain the unrelated oneshot, Jet Leg, which is included in the tankoubon for this series.