This is the least interesting building in Harpers Ferry and the most important one to get right. The visitor center sits up on Cavalier Heights at 171 Shoreline Drive, directly off US 340, about a mile and a half from the lower town by road and several hundred feet above it. It holds the park's main car park, the fee station, restrooms, an information desk, a bookshop and the stop where the shuttle bus leaves for the bottom of the hill every ten to fifteen minutes.
The reason it exists is that the historic lower town has essentially nowhere to leave a car. What parking exists down there belongs to residents, to deliveries, and to a handful of small private lots that fill early and are not a plan. The National Park Service has spent decades building a system to keep vehicles out, and the system is this building plus the bus.
The trap is satellite navigation, and the park warns about it on its own directions page. Ask a phone for "Harpers Ferry" and it will happily route you into the town itself, or down into the lower town, or to the park's headquarters building, none of which has visitor parking. People discover this at a junction on a narrow street with a queue behind them. Enter the Shoreline Drive address instead and ignore your map's opinion when you reach the turn.
Get here early on a busy day. The lot fills by the middle of the morning on summer and autumn Saturdays, and when it does, the shuttle queue builds behind it. The first bus of the day leaves at nine, and taking it is the single cheapest upgrade available to a visit here: you get the lower town before the coaches arrive.
Three things worth doing inside before you get on the bus. Ask at the desk what is open, because lower-town exhibits close and reopen. Look at the relief map, which explains the terrain better than any view does. And use the restrooms, because the ones in the lower town are busier.