From the site:
"These findings come from a survey conducted by Princeton Survey Research Associates International between January 2-5, 2014. The survey was conducted among a nationally representative sample of 1,005 adults ages 18 and older living in the continental United States. Interviews were conducted by landline (500) and cell phone (505, including 268 without a landline phone), and were done in English and Spanish. Statistical results are weighted to correct known demographic discrepancies. The margin of error for the full sample is plus or minus 3.4 percentage points."
They were asked, "During the past 12 months, about how many books did you read either all or part of the way through? Please include any print, electronic, or audiobooks you may have read or listened to."
So an answer of "one" can mean "I read one book part of the way through." That's also assuming that this question is being answered honestly, without the respondent fearing the stigma of answering "none."