Sites such as US gov sites have articles in the public domain.  When
using these articles in eg e-books, do I have to include the
attribution or can I just use the text?

Hello zenotopic~

In most cases, publications produced by the U.S. Federal government
are in the public domain. However, this isn't *always* the case, so
you'll need to search for copyright notices in whatever individual
publications you want to use. If you can't find any reference to
copyright, you can't assume that the publication is in the public
domain. You'll need to  write directly to whatever branch has
published the material, requesting permission to reprint, and
explaining whether or not the eBook you have in mind will be sold for
profit.

This advice is backed up by The Federal Citizen Information Center,
which distributes a wide variety of government literature:

http://www.pueblo.gsa.gov/faq/faq-pubs.htm#dist


Kind regards,
Kriswrite

RESEARCH STRATEGY:
Researcher's personal knowledge
Searching .gov sites