You, as a DHP publisher, are responsible for respecting copyrights. If you use another work within your own, make sure that you have the proper copyright permissions.
For works published since 1978, copyright protection lasts for the lifetime of the author plus 70 years.
For works before 1978, the copyright duration varies. It is always best to research a work to determine whether it is still protected. The U.S. Copyright Office can help you do this.
Many works are in the public domain (particularly older works) because their copyright has expired or they don’t qualify for protection. You may use these unreservedly. Make sure that the edition or translation that you want to use is not copyrighted by the editor or translator.
The law provides specific exceptions to the use of protected work. The fair use clause of United States copyright law allows limited portions of copyrighted work to be used for criticism or academic purposes. However, genres like “fan fiction” are not considered fair use and do violate copyright law. Since it is sometimes difficult to determine, it is best to obtain permission to use copyrighted material if there is any doubt at all.
Images are protected by copyright as well as the written word. Even if a certain work of art is in the public domain, the copyright for the actual image that you have may be held by the photographer who took that particular image. You need the copyright holder's permission to use the work.
Many online resources contain information about works in the public domain; some sites provide galleries of images that may be used. It is worthwhile to do the research to ensure that your book or project does not violate any copyright laws, especially older works.