I agree with Wayne's thoughts about the scope of permission. That is, if permission is given for a newsletter, it probably would not extend to another newsletter or book. A compilation of the newsletters strikes me as within a permission for a newsletter. When asking for or giving copyright permissions, it is prudent to do it in writing and be specific and clear about the scope of permission.

If you cannot find the copyright owner, then it might be what is called an orphan work (see http://www.copyright.gov/orphan/, http://en.wikipedia.org/wiki/Orphan_works, and http://www.publicknowledge.org/issues/ow for more information about orphan works).

There is no easy solution for the orphan works problem yet. One way to work reduce or avoid the problem is to look for Creative Commons-licensed works or, if you are comfortable with it, adding such licenses to your own works. Giving permission in advance in this way means the work could be used, even if the copyright holder cannot be found. More information on Creative Commons licenses can be found at www.creativecommons.org or in my Numismatist article from a few months ago.