Classic
or timely wisdom and art
Would
be fresh and valued even if first produced today
The
most widely acclaimed public domain exemplars are already
online
Current,
directly applicable knowledge
Much
(though not all) public domain informative literature now
outdated (especially if one only looks pre-1923)
Entertainment
Important,
but hard to get grants for
History
and documentation
Vast
majority of interesting public domain
text
Includes
literature, art, essays, science, etc., that are valuable
primarily in a historical context