The National Park Service explains decisions, documents information, and shares knowledge through a variety of publications, many of which are available online. This online library includes both contemporary and historical reports with new documents added regularly.
They include periodicals like
Browse the virtual stacks by topic:
History, Culture, and Historic Preservation
National Trails Systems
In addition, there are a number of public databases that may be searched by casual browsers or serious researchers.
National Register of Historic Places
Natural Resource Inventories of Parks