Different amplitude in time-domain vs frequency-domain
How can it be that the amplitude is −0.5


Primarily sloppy notation in the book.

The frequency domain representation of a signal uses complex numbers, i.e. you would need two real numbers to describe it. Either Amplitude and Phase or Real and Imaginary Part. The correct way to describe this would 


Amplitude is $0.5$ and the phase is $\pi$, or
Real part is $-0.5$ and the Imaginary part is $0$


The whole notion of negative/positive doesn't make sense for a complex number, so you can't think about this that way.