In Stout Fellow: A Guide Through Nero Wolfe’s World, published in 2003, O E McBride lists 60 words Rex Stout uses that “fall outside the average vocabulary”. Visitors to this site mostly have vocabularies that also fit this description and so will be easy with words such as egregious, fatuous, mendacity and sapient.
However, few will ever have come across abditory, which leads the alphabetical listing. It’s a hiding place, from Latin abditorium, a hiding place, whose source is abdere, to put away or hide. It appears in the story Instead of Evidence, in which explosive devices were found in an abditory in a factory.
The Oxford English Dictionary notes its first example from 1658, but it has never been in common use. Oddly, it is now more often employed than at any time in its history, not only because of Rex Stout but also by SF and fantasy authors, who have occasionally found it useful to help build a sense of otherness:
That abditory, the one in the buffet at home, contained a set of spare passports and other papers that might be useful under extraordinary circumstances. Other abditories, like the compartment under the front hall stairway, contained survival kits, or weapons, or money, or things as prosaic as the emergency roll of toilet paper.
The Crimson Sky, by Joel Rosenberg, 1998.
[Thanks to Art Scott for telling me about this word.]