I'm going to demand a cite for the claim that only about 15% of the U.S. is literate.
I'll also note that there's a distinction to be kept in mind between literacy (mastery of the artificial skill of using the written word) and fluency speaking one's native tongue (a skill all mentally healthy human beings pick up naturally).
First, http://nces.ed.gov/naal/ is the organization I was doing my reading with. Their surveys span the last 20 years. There's a 200-page PDF describing the surveys.
Second, "literacy" is, as you point out, a nuanced term. NAAL has broken it out into 3 categories with 4 rankings possible from Below Basic to Proficient. Based on my reading of the survey questions, I drew the line of "literate" as "proficient".
So, http://nces.ed.gov/naal/kf_demographics.asp is a summary page, and "
Percentage of adults in each prose, document, and quantitative literacy level: 1992 and 2003" is the graph.
You can see that the percentages are not collected into a single number. I made the ENTIRELY GROSS assumption that proficiency in one area probably leaks into the other areas. I would like to calculate the actual "Total proficiency" score based on the data though.
I'll also note that there's a distinction to be kept in mind between literacy (mastery of the artificial skill of using the written word) and fluency speaking one's native tongue (a skill all mentally healthy human beings pick up naturally).