I'm afraid that there is no copyright standard, and only certain
works are protected under the Life + 70 rule. Having said that, a
good summary of what is and is not in the public domain can be found
on this website:
http://www.unc.edu/~unclng/public-d.htm

Many images of Mark Twain were published during his lifetime and are,
therefore, not protected under copyright. However, any object, such
an image or manuscript facsimile, requires another level of credit.
These are often owned by individuals and institutions, and these
individuals and institutions will need to grant permission for an
object in their collection to appear in any sort of publication.

Here is another website with useful resources regarding copyright and
permissions:
http://www.aaupnet.org/aboutup/issues/copyright/index.html

Laura Cerruti
Acquisitions Editor
University of California Press