The usual easy rule to deciding whether a particular book is under copyright (at least in the U.S.) is to examine its publication date: works published before 1923 are in the public domain. But there are many works published before 1964 whose copyright notices were not renewed, and these works are also in the public domain. Normally, to determine if the copyright was renewed, you needed to do a search in the Library of Congress for a copyright renewal, and this normally is only done for a fee.
Stanford has now created a searchable index that can be used to find these renewal records for potentially public domain works:
This would seem to be very useful.
Technorati Tags: Public Domain