Quantitative Trading Fundamentals
I discovered Memlabs from their excellent YouTube content, which is completely free and full of practical advice and useful code examples.
I recently completed the Quantitative Trading Fundamentals course and was surprised at the breadth of content covered. While I have studied some of this material in the past, it's been a few years since I've been on the tools, so the course was an excellent refresher to bring me up to speed. I found the concepts were explained well, in an approachable manner. I will be applying what I have learned to developing my quantitative research skills and working towards getting my first algorithm live.
If you're new, this is an excellent place to start. If you're like me, with some experience, but have never worked in the industry, there are plenty of gems here that might otherwise take you a while to discover yourself.