What's the "one book" in your discipline?

Today in the post about mid-level backend engineering courses, someone offered "just buy this book" (Designing Data-Intensive Applications) and other commenters echoed support for the book. A few weeks ago in a thread about ML someone commented to the effect of "Everything you need to know in ML is in this book" (Deep Learning with Python) and again people chimed in supporting it. Maybe those two examples are actually off base, but I love the idea of the "one book" where you can get a decently complete picture of a given subject and level of expertise. So HN, what's your discipline and "one book"?