After failed Google and newspaper database searches for fullbooks.com, I did a "Whois" search, which is a way to find out who has registered a domain name. You can perform searches like these at http://www.networksolutions.com/whois/index.jsp

It turns out fullbooks.com is registered to Advameg, a company which a google search shows it to provide a variety of web content, including things such as the "Internet FAQ Archive." It appears that one of their products is licensing full-text books to be displayed online. There's a discussion of the relationship between Advameg and Thomson Gale on this library blog: http://lbr.library-blogs.net/how_things_are_made.htm
However, a search of various business database tools did not turn up a listing for the company Advameg, so it's hard to tell exactly who they are.

Beyond that, I was unable to determine whether all books on the site are in the public domain (although my hunch is probably yes), or why there are multiple versions of some titles. To find out more, you may be able to contact Advameg: the contact listed by the WHOIS search is:

Mazur, Lech
Advameg, Inc.
4 Knollwood Drive
Flossmoor, Illinois 60422

I hope this is helpful!