This is Masterton's first, and still one of his best. Masterton has an absolute gift for selling the preposterous, and making it entirely believable. He does so through dialogue and characterization, and I've never seen anyone do it better.
Karen Tandy visits her old boyfriend Harry Erskine, occult mavin and low-budget tarot reader to wealthy old ladies, because of a unique problem she's developed - a tumor on her neck, which to all intents and purposes appears to be a fetus. The doctors seem unable to remove it, and Harry starts experiencing paranormal disturbances after Karen comes to him for help. He, and a few initially skeptical doctors, reluctantly come to the conclusion that Karen Tandy is harboring the fetus of a powerful centuries-old medicine man about to be reborn - whose birth would first claim the life of Karen, and after, the entire white race, with his vengeful sorcery. What's modern science to do, against such a supernatural adversary? Why, fight fire with fire, of course - get another medicine man.
It's absolutely amazing that this piece works, but it's really great. Masterton never cracks a smile (until the very end), playing the situation up for real and sucking you into it so you believe it. The characters are fabulous, especially Karen, Harry - who appeared in the semi-sequel, The Djinn - and John Singing Rock, the rival medicine man to the rescue.
Masterton's stories almost always end on a lighter note, with the deliberate inclusion of a solution that is almost a joke, but the technique works because he's cluing his audience in to the fact that he realizes how silly it all is - he just wanted to show you he could make you believe it - and the concluding laughter he provokes is welcome and sympathetic, not denigrating the finely written novel at all.
The all-star movie made from this book in the late-'70s is worth a look. It's a faithful adaptation, though it doesn't work quite as well as the book due to some severe special effects deficits and a crummy musical score.