Works first published before January 1, 1923 with proper copyright notice entered the public domain no later than 75 years from the date copyright was first secured. Hence, all works whose copyrights were secured in the US before 1923 are now in the public domain, regardless of where they were published. (This is the rule Project Gutenberg uses most often)
Works published and copyrighted 1923-1977 retain copyright for 95 years. No such works will enter the public domain until 2019 unless one of the other rules applies.
So ... what are the best books published before 1923? Which of these are available online, and which should really be made available?
Seems like Wackernagel's Vorlesungen über Syntax: mit besonderer Berücksichtigung von Griechisch, Lateinisch und Deutsch is one thing that really should be scanned. What else?