The Dominica Museum, on the waterfront opposite the cruise ship berth, is the best starting point for understanding the island. The upstairs gallery walks visitors through Kalinago pre-Columbian society, the French and British colonial battles, slavery and emancipation, the maroon resistance fighters of the interior, and the island's twentieth-century politics — concise, well-curated, and grounded in the specific.
The museum is small (one large room over the ground floor) and admission is cheap. Allow an hour or two depending on how much of the text you want to read. Closed Sundays.