In the USA, the two most important cases were 1.
Feist Publications, Inc. vs. Rural Telephone Service Co. of 1991, where the concept of "threshold of originality" was addressed, and 2.
Bridgeman Art Library v. Corel Corp., which held that photographs or digital facsimiles of two-dimensional public domain originals lack sufficient originality for copyright protection. As of January 1, 2003, all unpublished work of known authors dead over 70 years entered the US public domain. Keep in mind that works
first published between 1923 and 1977 might be protected for 95 years from the date of publication, and works first published between 1978 and 2002 could be under copyright until 2048 - even if they are works of authors dead for centuries.