I read this post by Marc Andreessen some time ago and it nicely articulates what happens as you start a company and take it to market both in terms of the ups and downs.  I think his point on product/market fit is especially insightful and knowing you are on to something when the market begins to pull your product.  Read the whole thing.

I am (slowly) reading "The Four Steps to the Epiphany" and Blank’s market-centric process around Customer Discovery & Customer Validation ties nicely into Marc’s thinking (guess that’s why he recommended it).

I also think Brad Feld did a nice job of summarizing a key part of starting and growing a business in this piece on horizontal solutions finding their vertical applications based on market factors not marketing ones (he also does a nice job of covering what I see as "natural points of friction" between engineering, product marketing, and sales).