Use reference sources in your field to determine seminal works
Use a database such as Williams WorldCat to determine dates of different editions and translations
If your topic is very current or new developments happen frequently:
It matters which discipline; science papers need up-to-the-minute information, while social studies projects can use works published years ago; humanities papers often rely on sources decades or centuries old. Be sure to use reference works to find out which are the seminal works in your area.