One of the cool things about The Martian is "seeing" physics in action somewhere other than our planet. In most physics examples, we have things like friction or air resistance to contend with; after all, friction is everywhere. When we were first learning that an object with velocity but absolutely no net force acting on it would just keep moving without slowing down, it didn't seem to make sense at first (at least it didn't to me)--because every object we'd seen moving had been experiencing friction or air resistance.
But space is a pretty good vacuum; there's almost nothing out there to brush against and create friction. In the novel, Hermes experiences constant acceleration on its journey from Mars back to Earth, and, as a result, its final velocity was huge. Nothing was really slowing it down, so it just gathered more and more speed until the month before it was the reach Earth, as at that point it was traveling so quickly it need that entire month to decelerate enough to slow down to Earth's speed. Cutting the acceleration wouldn't stop the craft's velocity; it would continue traveling at its speed until it began to accelerate in the opposite direction.
Of course (spoilers) Hermes doesn't decelerate, instead deciding to do the "Rich Purnell Maneuver" and continue to accelerate past and around Earth, using the pull of gravity to adjust their course and head back to Mars. Its constant-thrust ion engine, which allowed Hermes to constantly accelerate, made it the spacecraft thing fast enough to get to Mars in time to bring food to Mark before he starved.