Ask HN: What one learning resource has taught you the most about startups?

Trying to learn as much as possible about how to start and grow companies but only want to study the from the best - what's helped all of you the most?

I recently finished http://startupclass.samaltman.com

Could be a book, course, podcast, blog post - anything - as long as it's something you can link to. Hopefully we can all benefit together