Pineland Farms in New Gloucester (about 20 minutes from Libby Hill) also has a much larger permanent orienteering course and their website has some good info about orienteering in general.
Up North Orienteers is an orienteering club in New Hampshire. They have an awesome website, with lots of great links to learn more about orienteering. Their family-friendly club hosts events for all ages and skill levels, posted on their club calendar.
The US Orienteering Federation is the “gold standard” for orienteering information or to find a club in your area (there are several in Massachusetts and around New England). Orienteering for the Young is part of the USOF’s website and a “must visit” if you have kids or are involved in youth education.
Wikipedia: Orienteering – has a good basic orienteering overview with other resources linked.