Second Horseman Out of Eden — George C. Chesbro

Christmas time usually finds Mongo in a jolly mood; he and his brother Garth customarily celebrate the season by granting the wishes of needy children. But there is no joy in this year's task - the Frederickson brothers must race the clock to bestow the gift of freedom on a little girl lost in a sinister 'secret place.'
The only clues are traces of exotic soil found in the creases of a letter to Santa and a cryptic reference that suggests the child's tormentor might be a man known all too well to the detectives - a renegade televangelist who is also a deadly white-power zealot.
A desperate search leads the brothers from the New York Botanical Gardens to the opulent Manhattan suite of one of the world's richest and most powerful men - and into an eerie, futuristic world-within-a-world. Here Mongo must face his ultimate challenge - to defeat the murderous giant who has dogged his every footstep... while seeking to avert a global holocaust that may be just a heartbeat away.