Rosemary's Baby by Ira Levin

Trade paperback format.  The classic itself.  Chilling and creepy as ever!

Rosemary Woodhouse and her struggling actor husband Guy move into the Bramford, an old New York City apartment building with an ominous reputation and mostly elderly residents. Neighbors Roman and Minnie Castavet soon come nosing around to welcome the Woodhouses to the building, and despite Rosemary's reservations about their eccentricity and the weird noises that she keeps hearing, her husband takes a shine to them. Shortly after Guy lands a plum Broadway role, Rosemary becomes pregnant—and the Castavets start taking a special interest in her welfare. As the sickened Rosemary becomes increasingly isolated, she begins to suspect that the Castavets' circle is not what it seems...