The official US government portal for all government publications is Publications.USA.gov. Publications which are available in print can be mailed for free to any address within the US. A digital copy can also be downloaded for anyone who wants it, including for those who live outside the US.
Publication categories include: Animals, Cars, Computers, Consumer Protection, Education, Employment, Family, Federal Programs, Food, Going Green, Health, History, Housing, Money, Small Business, and Travel.
As official government publications, most if not all are probably in the public domain. See Free IP – Public Domain. See also Free Samples, and Government Freebies.
Most Commented