It's not in Poughkeepsie (almost literally on the border in Wappingers Falls), but the one I know the best is called The Bookworm. It's on Route 376 about a mile south of the Inn at the Falls, and before you get to the airport. There may be others but that's one I know, and it's very very close to Poughkeepsie.
It's a small store but a good one, and book enthusiasts can probably appreciate it.