What's the single greatest resource you know about marketing?

With the rate at which paid ads on social platforms are changing what's the best way to keep up with how to best optimise your ad? 

Jef Lippiatt

Jef Lippiatt, Owner at Startup Chucktown

I've a big fan of the book "Punk Marketing", it is a not a super recent book, but very interesting.

Also, I'm a huge fan of the smart phone app called "Primer". It is a Google app, but it focuses on marketing, strategy, design and other related topics. Each lesson is a 5 to 10 minute lesson that is interactive and engaging.