I did a lot of research into Public Domain and international copyright because I was fed up with the whole "it may be PD in the US but that doesn't mean it's PD elsewhere," thing. The information I'm about to give only applies to work by US authors originally published in the US, determined to be in the US public domain, and that does not contain any supplemental material from later editions.
If an American writer first publishes a book with a US publishing house and that book is in the public domain in the US then the book is also in the public domain of every signatory country of the Berne Convention--which is just about every country
other than the US--due to the "rule of the shorter term." This rule states that signatories will consider work to be in their own public domain if the work is public domain in its country of origin. A complete list of countries is located here:
http://www.copyrightaid.co.uk/copyri...on_signatories
I'm currently battling Createspace over this matter because they are claiming that US public domain material cannot be sold in their newly-offered European Distribution Channel because it may not be public domain in the countries that channel covers--UK, France, Germany, Spain, and Italy. I'm pointing out to Createspace that all of those countries signed the Berne Convention and adhere to the completely logical and simple "rule of the shorter term." Any US-generated, US-published public domain work is also public domain in every country Createspace distributes to.
There are obviously some complications that arise when books are written or published across multiple countries or the edition in question uses material or translations that may still be under copyright in whatever country that supplementary material was created.
In my opinion, the perceived murky nature of copyright and international copyright on possible public domain material is not protecting IP rights as much as it is ensuring that many lesser-known works will be lost and their creators forgotten.