Colin, with the utmost of respect to you, if you can't make a YP ad profitable you should only be disappointed with yourself for either paying too much for the ad or having an ad that's crap and doesn't make people want to call YOU rather than the 30 other firms in there.
It's not the book that's at fault, for christ's sake it's like shooting fish in a barrel!! Mrs Jones wants her carpet cleaned, Mrs Jones picks up the YP, Mrs Jones looks for a "good" firm to ring. It's just a case of appearing more "good" than those around you.
What constitutes "good" is a very open-ended subject
Don't expect to become a millionaire from YP if you price higher and prefer quality clients. If you can run around doing 6 jobs a day for cheapo prices you'll get more jobs from the YP. Square peg round hole!
10 years ago millions of people used it, now only a fraction of that number do due to the tinternet and all. But many older people still don't have computers/net connections and still do things the traditional way.
With a GOOD advert the YP is capable of between 5k and 10k a year depending on your area, your phone sales skill and strength of competition. You can get that from an £800 ad.