I have not heard of the US Yellowpages, just the AT&T Yellow pages or Yellow Book.
I know some people that have to be in them as in their industries not enough people are searching online yet, more and more each year but not enough yet.
Each year they get larger ads and the increase of work is worth it to them, one has a website but when asking customers how they found him, almost all said the phone book.
I would ask your customers how they found you and see how many actually use the phone book, if the numbers are up there, it might not be a bad investment but you need the right book, too many copycats and worthless books out there that try to act like they are the real deal.
Good luck.