As a financial professional, I recently read famed bond investor and head of Oaktree Capital Howard Marks' "The Most Important Thing: Uncommon Sense for the Thoughtful Investor"
There are actually 18 Most Important Things contained in the book.

While the book is about investing, I think poker players can extend many of the lessons in the book to poker by thinking outside the box a little. Heck you may learn some about investing too!

To give an example of a few of the chapters that particularly apply to poker.
Each can be read as "The Most Important Thing is":
Second Level Thinking
Understanding Risk
Controlling Risk
Patient Opportunism
Appreciating the Role of Luck

As background, I first got involved in poker after reading a short article in Smart Money magazine in 1998. They had done a short article on Sklansky's "The Theory of Poker" and discussed how similar poker is to investing. I read Sklansky's book and the rest is history.

To me, playing good poker and practicing solid investing have many parallels.

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"Sure, I've been called a xenophobe, but the truth is, I'm not. I honestly just feel that America is the best country and the other countries aren't as good. That used to be called patriotism" ~ Kenny Powers

I'm guessing that's not the same guy as "Mr Nice" Howard Marks, the infamous international drug smuggler ?

... mind you, I'm guessing he`d know alot about pot control.

Lol.
He'd also be particularly good at using deception!

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"Sure, I've been called a xenophobe, but the truth is, I'm not. I honestly just feel that America is the best country and the other countries aren't as good. That used to be called patriotism" ~ Kenny Powers