since you saw the suggestion in a US book, how about a US answer.
FAA has visits by pilots (not just GA) as official policy called Operation Raincheck. Since 9/11/01 the availability has been spotty, a definate "not no but h*ll no" at some facilities and an "okay, but" at others.
There are visists and there are visits...Raincheck visits you get to watch the control floor of the center from a vultures row above. It is better if you can scrape some aquaintance and plug in a position for awhile, can be a real show! GA control towers w/o radar etc can be more exciting than the big ones too... though I seem to be there when the big tower has a bomb threat on an incoming a/c and at the GA one when radios go u/s and they control with cell phones via landline (it worked, by the way)