Dumb Witness is Christie’s 1937 book.

It is the last book to feature the character of Hastings until the final Poirot novel, 1975’s Curtain: Poirot’s Last Case, which he also narrates. …

In the New York Times, this novel was not considered Mrs Christie’s best, but “she has produced a much-better-than-average thriller nevertheless”…

I’d agree.

It’s standard Christie. But not quite as neatly solved as most of her other books.

In this one, a wealthy spinster dies — before Poirot could get there.

Despite having no paying client, the Belgian sleuth suspects the death by natural causes is actually murder. And stays to solve the crime anyway.