Merck Manual of Diagnosis and Therapy
The Merck Manual of Diagnosis and Therapy, referred to as The Merck Manual, is the world's best-selling medical textbook, and the oldest continuously published English language medical textbook. First published in 1899, the current print edition of the book, the 20th Edition, was published in 2018.
Source: Wikipedia — Merck Manual of Diagnosis and Therapy (CC BY-SA 4.0)