For your own safety, do not destroy Vital Testing Apparatus

The Powerhouse by John Buchan is a 1916 thriller mystery about an international secret criminal organization that's absolutely laughable in light of (1) the later course of history and (2) the development of the genre. Readable, pleasant narration, and quite a turn of phrase, but insubstantial.

The Patient in Room 18 by Mignon G. Eberhart is set in a private hospital in the American Midwest in 1929, and that made it interesting at first. It has some gobsmacking passages that it doesn't seem to know are racist ("This other guy was obviously wrong to be prejudiced against this mixed race woman but she is obviously fashionable and lazy because of her Black ancestry" - the enlightened detective). The plot relies on a witness to the first murder waiting a week, then deciding to spill his guts to the narrator in a clump of bushes where anybody could overhear, then refusing to say who did it and running away to get murdered while the narrator is just like "Huh!"