The line that changed how I look at things: competition is not proof of a good market, it is usually proof the idea was obvious. I had been treating a crowded space as reassuring, like other people had checked it for me.
The other one I keep coming back to is the question about what you believe that almost nobody agrees with you on. It is a hard question to answer honestly and I did not have a good answer the first time.
It is short and it is written to convince you rather than to walk you through the evidence, so I would not take all of it at face value. Worth it for those two ideas alone.