If a listener reports s/he has on a station, it will get reported to Arbitron. There are certain minimal levels before a station shows up in the book, however. There are also break-outs by county which could help suburban stations overshadowed by large market stations show up in an individual county.
Someone once told me that once you subscribe to Arbitron, any station can make itself look good, somehow, because there are so many ways to divide up the audience.
For example: in one county, weekdays from 10am-3pm, with men 45-54, a station might be top five. Overall in the book, it could be below number 20.
But, the more specific the information from Arbitron (i.e. you pay for their packages that provide this sort of information) the more expensive it is.