I think the most succinct intro I have found is Donald Wheeler's Understanding Variation. I've long wanted to write an open article series for someone like you

I think the most succinct intro I have found is Donald Wheeler's Understanding Variation. I've long wanted to write an open article series for someone like you but never gotten around to it. There's so much information out there, but you sort of have to piece it together on your own, which is suboptimal.