Benjamin Graham's The Intelligent Investor elucidates the principles of value investing as opposed to risky gambling. This is a book recommended by Warren Buffett too.
I tried to get through this, but after one or two chapters it seemed really dry, and somewhat outdated for current markets. I've read dry material on financial markets before, but jeez. Did you actually learn much from it? If anyone wants to post a tl;dr summary, it would be appreciated.
It's written in simple language (and modern editions come with commentary updating it ) if you cant read that book then maybe you should not invest in shares at all.