What are the best books for developing a solid understanding of fundamentals of Economics?
Read Hal Varian's textbook "Microeconomic Analysis". It's brief, yet a standard text in a 4th year or graduate course in advanced micro.  Then grab a good book on mathematical economics that's actually well written, and includes suggestions on how to develop good mathematical intuition, not just a bunch of recipes on how to solve problems.  For example, you can learn how to use determinants in matrix algebra, but how many actually know what a determinant is, both algebraically and geometrically? That's the blood, sweat, and tears bit, I think.   I suggest the two volumes by Knut Sydsaeter (check Amazon)

You may also want to learn some advanced macroeconomic theory, and some spatial, urban and resource economics. 

Take a good course in econometrics, and learn a programming language like Python or Julia so your are not always beholden to other people to provide you with data, which may be dirty or faulty, but you won't know it. 

Watch lots of Youtube videos on economics, esp. controversial topics. Take some cool courses on Coursera or other MOOCS. They are often from some of the top universities in the world, and they are free. 

OK, so now you have some basic tools; if you enjoy using them, and find that real work gets done, why not go to graduate school? 

Then go practice for 20 years, and keep in mind that some aspects of economics may just be bunk anyway, so stay modest, don't get too overly seduced by beautiful models.