Here are some additional image credits from the copyright page of the book (with the links spelled out):

The pocket watch and barbed wire snippet were posted to Clker by OCAL. The site is a very useful source of public domain clip art.

The map of North America is adapted from the CIA World Factbook. As with most US Government publications, the original is in the public domain.

Anything in the public domain may be used and modified with no restrictions, and no credit given. (However, it's still polite to give credit.)

A creative work is "public domain" either if the copyright has expired (e.g. in the US, a book printed in the US prior to 1923) or if the creator has specifically relinquished all rights and placed it in the public domain. Many countries grant copyright for the life of an author plus 70 years (in Europe, Australia, etc.) or 50 years (in Canada and elsewhere).

Note that images and sound recordings generally have their own complicated rules for when copyright expires.

Some useful links:

• John Mark Ockerbloom's "Online Books Page" site includes lots of details
• Public Domain Sherpa is thorough and well-written
• Canadian Public Domain Flowchart (pdf)
• Creative Commons offers a public domain license