If there is one book that I should read to truly understand the game, what is it?

In short, no.
No single book will help you understand chess. But there are two books that come close to doing that. Both are structured in a way that will help you see how strong players think during a game. Think Like a Super-GM and Lessons with a Grandmaster.

Lessons with a Grandmaster
Lessons With a Grandmaster bridges the gap between Grandmasters and amateurs, and it explains the differences in their thinking processes, the way they analyze and calculate, and how they see a chess position. A remarkable book and a refreshing concept that combines the chess expertise of GM Boris Gulko, and the scientific curiosity of Dr. Joel Sneed, a clinical psychologist. They have gone over 25 games played by GM Gulko against the world’s best at the time of his peak strength, Kasparov, Karpov, Miles, Yusupov, Korchnoi, and others. Each game is a conversation, a training session between the two authors, during which they share their thoughts on the positions, often vastly different, allowing the reader to see why and how a Grandmaster is able to be more efficient at finding the best moves. Lessons with a Grandmaster is like having 25 free lessons with one of the top Grandmasters.

Think Like a Super-GM
Wanna know how strong players think when analyzing positions? Now you can. Think Like a Super-GM is, simply put, a collection of problems solved by players of varying strength. Their thinking processes are written down, so that the reader can see exactly how they think, how long it takes them to spot certain features of the position, come up with candidates, or calculate the variations. I think it’s a splendid idea and I wish there were multiple volumes of the book since the 40 or so positions presented had left me wanting more.

Build repertoires
Serious chess study should also include openings. Here is what I use to study openings and build repertoires effortlessly https://chessbook.com/chessreads