A Study in Scarlet by Sir Arthur Conan Doyle
This is the first Sherlock Holmes mystery, written purportedly while Conan Doyle was not-busy being an unsuccessful doctor.
It gives us the origins of how Holmes and Watson met (entirely invalidating the Young Sherlock Holmes movie from the 80’s). It is odd in that Holmes is entirely absent from the middle section of the book. This section is a somewhat abrupt jump back in time to give some background story on the mystery in question.
The middle section is also quite an unflattering portrayal of Brigham Young, and the Latter-day Saints colony in Utah of the time. I can’t imagine the Mormons were (or are) big fans of Conan Doyle.