I think it would depend on what you’re into, but these are my picks:

Quantum Electrodynamics (Feynman): A summary of how almost EVERYTHING works (albeit explained simplistically).

Poor Charlie's Almanack: The Wit and Wisdom of Charles T. Munger (great insights / worldly wisdom from one of the world’s greatest investors).

The Vital Question: Energy, Evolution, and the Origins of Complex Life

The Princeton Companion to Mathematics (Great overview of mathematics).

Quantum Mechanics: Concepts and Applications (Great QM book).

Wooden on Leadership: How to Create a Winning Organization (Great leadership / management insights).

Why Does the World Exist?: An Existential Detective Story

Sapiens: A Brief History of Humankind (An extremely simplified historical overview of how we got here).

The Book of Disquiet (Serpent's Tail Classics) (Random journaling from one of the world’s greatest prose writers: Fernando Pessoa).

Code: The Hidden Language of Computer Hardware and Software (Excellent explanation on how computers work from bottom -> top).

The Machinery of Life (Includes excellent visuals in biology).

The Character of Physical Law (Great insights in physics by Feynman).

The Goal: A Process of Ongoing Improvement (Great book on business process improvement).

How to Win Friends & Influence People (Nothing groundbreaking, but still a worthy read).

Coders at Work: Reflections on the Craft of Programming (Great info and insights from some of the greatest software developers of all time).

The Periodic Table (Great read and some of the greatest prose writing you’ll ever encounter).

My discipline comes from a lifetime of sports and has carried over to my work life (even though I now do martial arts as a hobbyist instead of competing as a college athlete); many of these can be read as workout / health points but are actually applicable across your life.

It starts from the long-term goal: if you want to be extraordinary, you need to do things differently than the ordinary. People are going to give you a hard time about some of the changes you're making, so you need to be ready to ignore them and accept the fact that if you want to have a different level of discipline than others you need to do things differently.

Discipline is compound interest in your life - you are doing the little things the right way every day, and all those pennies are eventually going to add up into millions. This applies to food, working out, pushing on projects, learning new skills etc.

The best advice is to make it easy for yourself - pick a FEW, SIMPLE, things that you can control and start from there. Stop eating the same snack or having a glass of wine after work. Start waking up at 6 am. Then evolve to going to the gym from there. Build habits.

You also need to accept that you're not perfect, and no man is an island. This part was the hardest for me - I built up walls around myself (thinking that I needed to keep my habits exactly as they were since they had gotten me this far) and thus cut off opportunities and other people. Eventually I was able to be a bit more chilled out while still keeping the discipline in the right areas. I found that 7 habits of highly successful people really helped me; I also recommend "Wooden on Leadership" and "The Score Takes Care of Itself" as examples of champions putting this bottoms-up approach into practice over years and reaping the rewards.

There is no shortcut. There is nothing fancy. Control what you can control, don't worry about the rest of it, and trust in the process. Feel free to message me if I can be helpful