An updated version of Historical Statistics of the United States has arrived, the first new edition since 1993. It will live in the phone room. This source is handy for commonly-requested historical census information, especially that which is too recent for the Historical Census Browser web page.
City-level historical statistics can be hard to find, but Erin points out that there are sources for the population of St. Paul in the St. Paul Collection. Check in the St. Paul subject index under "Population".