Worlds of Maybe

Blurb:

(from Dell 1974)

WHAT IF the world had been at peace since 1916?
WHAT IF Carthage had not been destroyed by the Romans?
WHAT IF Columbis had discovered that the world was flat?
WHAT IF you could rent a planet where life had never evolved?
WHAT IF a time-quake could jumble past, present, and future?
WHAT IF you should slip into a parallel world to this that is nearly — but not quite — the same?