Lots of public domain books (03 Nov 2005)
Read the Google weblog post if you want, but the operative information is that you goto Google Print and use the date operator to restrict searches to public domain books.
If you are in the US you can use date:1500-1923 and, if you are outside the US, you can use date:1500-1846 (or proxy through a US host).
Update: fixed link - thanks Aaron.