Lupoff, Richard A. Circumpolar!
What if: The Earth were disk-shaped, with the North Hole at the center.
Summary: Two groups, American and German, travel to the other side.
Published: Simon & Schuster/Timescape 1984 (0671499416); Berkley 1985 (0425081931); Grafton 1988 (0586060669).
Lupoff, Richard A. Countersolar!
Summary: Albert Einstein races the Perons to counter-Earth.
Published: Arbor House 1987 (0877958580); Grafton 1988 (0586070966); Ace 1989 (0441117910).