The Plagiarized Sources and Site of Publication of Misappropriated Material
For a start, the nature of the original sources and the site in which the misappropriated material appears are significant. For example, if someone plagiarized material from a journal article and published it (perhaps in translation) in another journal, one could notify the editors of both journals and well as the author of the plagiarized work. Copyright violation might or might not be involved (since plagiarism includes theft of ideas, but only forms of expression and not ideas can be covered by copyright.
If the material were in the public domain, of course it would not be copyrighted, but still might have been plagiarized, in which case the publisher of the misappropriated work could still be notified.
I mention journal editors as the first group to inform, because journal editors have become more proactive in recent years. I think especially of the International Committee of Medical Journal Editors--see http://www.icmje.org/urm_main.html. However practices and expectations for medical journals are somewhat different from those for engineering journals. Therefore, if a case arises in the engineering literature, it would be better to consult something like the IEEE guidelines on publications, which unfortunately is hidden away in their in their PSPB Operations Manual (which you can download as a PDF from http://www.ieee.org/web/publications/rights/policies.html and then go to section 8.2. The IEEE also has a document on plagiarism that is accessible as a Web document at http://www.ieee.org/web/publications/rights/ID_Plagiarism.html.
The IEEE provides a flowchart of their procedure in responding to complaints of plagiarism at http://www.ieee.org/portal/cms_docs_iportals/iportals/publications/pubtoolsandpolicyinfo/Investigating_Plagiarism_Complaints.pdf. It shows that the produre is complex and so may take some time.
If either publication were funded by a government grant, the funding agency would want to know, but may be able to take action only if they funded the plagiarizer's work.
If journal editors, book publishers, etc. don't respond, and the perpetrator is a member of a college or university faculty, you could then send copies of the relevant text or documents to the research standards officer (who might have any of a variety of titles, including vice-president for research) at the perpetrator's institution.
In the case of plagiarism (unlike cases of data fabrication or falsification), there is not much danger of a perpetrator destroying the evidence, so you could even contact the perpetrator.
I hope that helps.