Based on TwoBitOperation's answer, I'm guessing its
Spoiler
I can keep you grounded
A car spoiler keeps it grounded
I can rile you up
Being spoiled makes people angry
I can facilitate your travel
Spoilers on plane wings help them fly
I can discourage you from going
If something is spoiled for you, you don't want to go as much
Everything regarding the hints and the forms is laid out in TwoBitOperation's answer, but in summary
Each form is a famous spoiler
Also, for the title
A reader should beware a spoiler when reading a book