What is the underlying concept behind Bandpass sampling?

Can you suggest me some books/webpages on Bandpass sampling?

I undestand that if the signal is restricted between $f_L$ and $f_H$, then the minimum bandwidth required is $2(f_H - f_L)$. But say the centre frequency is in GHz and the bandwidth is in MHz, then by this theory I would have the sampling frequency to be in a few MHz. So in case of GHz centre frequency signal(say 1 GHz), I collect the samples every microsecond. But the signal is changing every nanosecond. I did not understand this concept.