The library has a number of books on statistics, including books for statistics of various disciplines, programs like SPSS (PASW) or R, and more
One of the single best resources for learning about the US Population is the census. It is mandated by law that the Census Bureau conduct a nationwide census every ten years (the data from this effort creates the decennial census, which most people think of simply as 'the' census). There are also other census resources, including the American Community Survey, which are conducted more often.
For any census prior to 1990, access to the official US Census data is only available in print or microformat (the library has the Census from 1790-1990 in various formats; ask at the Reference Desk). There are also some unofficial sources linked below that allow you to analyze data from before 1990 electronically.