I started reading The Economist last year for this reason. Their Special Reports are similar to what you're looking for. The New Yorker also puts out minibook-l

I started reading The Economist last year for this reason. Their Special Reports are similar to what you're looking for. The New Yorker also puts out minibook-length articles. Nautilus is another mag that I've been happy with for more science-oriented stuff. They put out monthly issues with a cohesive theme.