The Public Domain is a special category of works that are outside of copyright (generically speaking, but this is not a universally true statement when factoring in other issues like Creative Commons that add to the heady stew that is the copyright issue). What qualifies for the public domain can be a confusing. Some examples include
If you get the sense that some of those numbers conflict, then you are on the right track. Any browsing through something like The Internet Archive or Project Gutenberg (both of which store large amounts of public domain works) will find some subset of works that appears to be copyrighted in every way except for the fact that it is not. Simply put, public domain status is not something that can be assumed because of some heuristic, and often requires a series of document checks except in the case of a few givens (Dracula, the book, is public domain for sure). One of the best tools to use is The US Copyright Office's own Catalog of Copyrights. If you see the "How to check" document linked above and up on the left sidebar, there are addresses you can write to (and send a fee to) in order to have someone double check. Using the aforelinked CoC to check the status of the book The Galactic Pot Healer, I can find that the copyright is currently held by Laura Coelho, Christopher Dick, and Isolde Hackett. And it was renewed in 1997. Since Philip K Dick, the author, died in 1982, that means the copyright will be up in 2052, assuming things do not otherwise change (and they will change, and no one will be able to predict how, unfortunately).
What does a work being in the public domain change? Depends on what you need, but maybe not a super whole lot. While you can make derivative works and quote whole public domain poems and short stories, and copy until your heart or Flex Account is content, this does not mean you are able to use the work without citing or giving credit (unless you wish to commit plagiarism). Just because Tom Sawyer is public domain does not mean that you can claim you wrote it.