I was at the same point about a year ago. I never really found a single primer, and I looked. What I realized is the industry is complex because it is fragmented by tons of companies doing their own thing, not because of any real inherent complexity.

I started following adexchanger.com pretty closely and I read through their archives, which helped a lot. Adexchanger is good because it gives you a chance to see what companies are doing, and how they sell themselves.