While a part of the brain, the part connected to your sight, thinks that you are still because you are looking at a book, another part, the part connected to your inner ear, thinks that you are moving because it senses motion.
People who stay inside ships might suffer from the same thing because their eyes can't see the motion. This is why those on deck are less likely to experience motion sickness.
After some time the brain can learn to adjust to such conflicts. This is why some people stop getting motion sickness after boarding a boat for some time.
This conflict in information reaching the brain triggers nausea as the brain starts believing that you have eaten something toxic that's making you hallucinate. See also what alcohol does to your brain.
Not all people get affected the same way by motion sickness. People who are more sensitive to motion sickness usually experience those symptoms fast.
If the road has many turns or if it's bumpy then motion sickness is more likely to occur. This is why reading in a train doesn't usually trigger motion sickness.
According to one study, certain genes can increase the chances of a person getting motion sickness.
People who sit in the back seat are more prone to motion sickness as their eyes can't see the road properly and so might send wrong signals to the brain.
Pregnant women are more likely to experience motion sickness than normal women.