The books in the Bonus Ebook folder are public domain.

In the United States, the following rules apply: 

Anything copyrighted prior to 1923 is in the public domain. (Practically speaking, this includes anything published prior to 1923, since publication without copyright put the work straight into the public domain. But note this possible exception in some western states for some 1909-1922 foreign works that were not published in the US before 1923.) Due to a 20-year copyright extension enacted in the US in 1998, copyrights from 1923 or later that are still in force will stay in effect through 2018 or longer. 
Certain works copyrighted in 1923 or later may already have entered the public domain. In particular, works published in the US before 1989 without proper copyright notice, and works published in the US before 1964 whose copyrights were not renewed, may have entered the public domain. However, works from 1923 or later that were originally published in countries outside the US may still be copyrighted regardless of whether they were printed with proper notice or renewed. To research whether a book's copyright has been renewed, or needed to be renewed, see this article. 
Works never published prior to 2003 (and never registered for copyright prior to 1978) are now in the public domain in the US if they are by authors who died more than 70 years before the most recent New Year's day. (For 2004, this means authors who died before 1934.) Although this new rule does not put any previously published material into the public domain, it may allow some long-lost manuscripts and collections of letters to be published online as "new" online books.