To get to the mountain follow the Appalachain Trail! Katahdin is just north of Millinocket, ME, in Baxter State Park. There are detailed directions and insructions for winter climbing on the state park web site: www.baxterstateparkauthority.com/ The park is awesome in the winter and offers full bore, hard alpine climbing as well as shorter, more friendly ice climbs. Expect to spend two days coming in and out as you are required to stay at the Roaring Brook Campground on your first and last night in the park.
In the summer the park rangers will hand out a "Technical Rock Climbing At Katahdin" paper which has general information (basic route descriptions with grades that are not all that helpful.) There is also an informal guide book with photographs and drawn lines of the routes along with descriptions from many of the first ascentionist. Be prepared to show your gear and lay out your intentions to the ranger on duty, who may or may not know something about climbing and may or may not be polite and respectful.
I'd like to hear more info on the Flatiron route. I have been in and out of the park for 35+ years in all season, even snowboarding down off the second cathedral way way back one winter, worked trail crew some, etc... and only recently started to technical climb on Katahdin. We did what we think is Flatiron route - trying to interpret some handwritten notes from the "bible" at the Ranger's station...Anyone have any route notes on this one they want to share?
The bottom line, for those interested, is that the park is remote to begin with and then you are remote while climbing. For people who think the rangers are being stiff, they don't know you from adam, and they are doing their jobs to keep you safe. The part can be really nepotistic, find someone who they know to take you in and then you are golden. It isn't that hard, there are a lot of climbers in NE who know the park...