You'll know already if there is somewhere in your house suitable for a bookcase to hide a door. For us, it was a weird little space with an angled ceiling and a window. It used to have a balustrade and was entirely useless, but one of the first things we did was wall it off and create an open study nook. That left it open to the living room, which was OK but not ideal, since keeping such a space tidy is a PITA. What makes it suitable for a secret room is that it is cantilevered out over the front door, so unless you're really paying attention when you come into the house, you won't know it's there. It's already surprised visitors, which is ridiculously satisfying.