basic principles. I thought this was an adequate article that introduces people to index funds and ETF's, although the focus is more towards index funds (which is fine). It provides several alternatives on how to do it. It is a bit lazy with such a strong focus on Vanguard, but provides enough alternatives (and there are more than what's in the article).

https://www.fool.com/investing/2018/05/19/the-beginners-guide-to-index-funds.aspx

Basic, but adequate for designing an investment plan. I have always believed that the actual saving money is the most significant and difficult part of building wealth - not what you invest in, but with tools like those in the article, it becomes even more significant.