Sicilian Defence (1. e4 c5) and how to develop it further by using different variations. It is easy to learn, it has hundreds of years of history and it is very popular in grandmaster tournaments.
I think the most common thing to implement is the perceptron algorithm from scratch.
I would recommend Pattern Recognition and Machine Learning by Christopher M. Bishop. It assumes no previous knowledge of pattern recognition or machine learning.
Learn You a Haskell for Great Good! is aimed at people who have experience in imperative programming languages but haven't programmed in a functional language before.
Esoteric programming languages are usually created to bend the boundaries of programming language design or prove something, not to be actually used to write software. It is hacker culture.
"That's been one of my mantras - focus and simplicity. Simple can be harder than complex: you have to work hard to get your thinking clean to make it simple. But it's worth it in the end because once you get there, you can move mountains."
Interactive and high-level languages are really fun to use. This reminds me of Sharp tools essay in The Mythical Man-Month (1975).
"The most important two tools for system programming today are two that were not used in OS/360 development almost a decade ago. They are still not widely used, but all evidence points to their power and applicability. They are (1) high-level language and (2) interactive programming. I am convinced that only inertia and sloth prevent the universal adoption of these tools; the technical difficulties are no longer valid excuses."
I started reading The Wealth of Nations by Adam Smith and I think I'll continue with Debt: The First 5,000 Years by David Graeber. Not really compexity economics, but classical economics and anthropology.
The Art of Electronics is on my technical reading list as well.
If you are into Clojure, you will probably enjoy Elements of Clojure by Zachary Tellman. It is not a tutorial, it's more like an attempt to bring tacit knowledge of idiomatic programming practices into light.