What value do you see in phone book advertising?
Though the majority of folks looking for anything (product or service) are researching things online, some market segments are more likely to grab the print phone book directory to find local business listings when at home.
Businesses that first come to mind are: the local pizzeria, plumber, tree service company and landscapers.
That said, it would help for you to assess the demographics of your current client base and conduct your own survey on phone book usage with them, as well as with a mixed audience at your next local networking event.
It is certainly worth checking out the prices/packages for print and online listings on Yellow Pages.com, as any hyperlinked online ads/listings help to drive better organic search engine rankings when people use the keywords "your area, landscaper" or "landscapers, your area".
Give me a shout directly if you'd like to discuss your specific marketing challenges/options. Best of luck in all your endeavors!