I was making my "all time favorite books" list, and I realized that all five lead to much more mental toughness, resiliency, critical thinking, and groundedness. Here you go, all incredibly worthy and potentially life-changing reads --
1. Principles by Ray Dalio [pdf]
It'll make you much smarter and more effective. Dalio built the largest hedge fund in the world and became a billionaire in the process. He outlines his methods of thinking, personal action, and teamwork/collaboration in Principles.
As a large bonus, it was originally started as an internal document only, so the amount of propaganda and trying to sell his way as best is very low. It's a masterpiece and a must-read.
2. The Tao Te Ching [discussion on translations and reading guidance; links within to 3 pdf versions]
It rapidly has become my second favorite book. It requires work, but there's an immense amount of lessons here. It helps you think of what your existence is, gives guidance on living in a grounded and pleasant way, helps relate to your own thoughts, and has some powerful practical insights on conducting a life well and running an organization well. Every time I open it and start going through it, I get something excellent out of it.
Incredibly powerful. In my opinion, it's the definitive book on heroism and what it takes to shape the course of history. It starts by laying out that every man has a religion, but Carlyle defines religion as what a person chooses to live by. It then looks at, "Where did those religions -- those codes of living -- come from?" His answer? Heroes. It's incredibly poetic and powerful.
4. Musashi by Eiji Yoshikawa [Amazon]
The true story of the raw unpolished youth rising to become the greatest swordsman of his age. It's a critical book for mastering yourself if you've ever struggled with putting your abstract potential together, if you've ever found yourself in conflict with mainstream values, if you've ever insisted on doing things your way even when it goes against natural law and leads to failure. It's also a heck of a swashbuckling story -- it was serialized in magazines for over a decade, so every chapter has rising and falling action and narrative arcs. It's a very important and good read.
Make sure it's Haddawy's Deluxe Translation -- I tried two or three others after I gave away my copy of Arabian Nights, and none of them did it for me. Arabian Nights contains an immense amount of practical wisdom wrapped in a fascinating narrative. But what's so interesting about it is that it's downright vulgar while being poetic. Most versions that made it into English were sanitized, but Haddawy's is... really, really raw. You get to see the dark sides of human nature on display in this book through tales and parables, which should help you put it just far enough away from your ego to recognize and correct your own mistakes.