I can recommend two sources that will provide you with a lot of information especially as this year is the anniversary of the founding of the National Park Service.
First the National Park Service's webpage which has numerous resources including lists of all their parks, their history and what they've achieved. http://www.nps.gov/index.htm
Another useful reference is National Geographic which has a number of resources about the Park collected here: http://www.nationalgeographic.com/power-of-parks/
I hope these help you, all the answers you need should be easily found on these sites.