Portals of Twilight (1981)

From the Introduction:

"This is the third product in the "Portals" series. The first two products, Portals of Torsh and Portals of Irontooth, deal with worlds in which the primary inhabitants were significantly different from those of a normal Advanced Dungeons & Dragons world. This product is slightly different in that the inhabitants are fairly normal, but the world itself is somewhat strange. Halfworld, the world of Portals of Twilight, always faces the same side toward the sun."

 From the web:

The third book in the "Portals series." Journeys of wonder and magic will be experienced when the Portals are traveled again in this newest release in the Portals series for AD&D. Portals of Twilight is set on the planet Halfworld; it is a world similar to our moon, half light and half shadow, with only a narrow strip of land that is habitable. This twilight region has evolved varied types of creatures which are accustomed to the climate. North Pole City can be visited also, as it is the center of learning and culture on Halfworld. Multiple encounters are given, as well as other major points of interest and new monster types.