The ruling in Bridgeman Art Library v. Corel (1999) made it more difficult
to claim a new copyright in reproductions of public domain images. See:
Bridgeman Art Library v. Corel
My understanding is that the ruling in Bridgeman v. Corel allows
reproductions of "slavish reproductions" of two-dimensional public
domain images like book illustrations. Assuming that the illustration in
Our Mark Twain is a "slavish reproduction" of the original illustration, it
would not qualify for a new copyright. Of course, someone wanting to
reproduce an image needs to be sure that it is a "slavish reproduction"
and has not been altered (e.g. has not been redrawn, colorized, or
otherwise changed) in a way that would allow an original copyright.
Bridgeman v. Corel may not be entirely relevant for book publication
because it only affects U.S. law. It is not recognized in the UK. If you
needed to be able to provide world rights, you might need to get a more
widely recognized clearance for use of the images.
There are many discussions of Bridgeman v. Corel available on the web: