the lean startup

is an excellent book by Eric Ries (isbn 978-0-670-92160-7). As usual I'm going to quote from a few pages:

What if we found ourselves building something that nobody wanted? In that case what did it matter if we did it on time and on budget?

I've come to believe that learning is the essential unit of progress for startups.

If you cannot fail, you cannot learn.

Qualitative learning is a necessary companion to quantitative testing.

Pay no attention to the eight people behind the curtain.

When I could think of nothing else to do, I was finally ready to turn to the last resort: talking to customers.

That which optimizes one part of the system necessarily undermines the system as a whole.

Large batches tend to grow over time.

The paradoxical Toyota proverb "Stop production so production never had to stop."

We routinely asked new engineers to make a change to the production environment on their first day... They would ask, "What will happen to me if I accidentally disrupt or stop the production process?" ... We told new hires, "If our production process is so fragile that you can break it on your very first day of work, shame on us for making it so easy to do so." If they did manage to break it, we immediately would have them lead the effort to fix the problem as well as the effort to prevent the next person from repeating the mistake.

Switching to validated learning feels worse before it feels better... the problems caused by the old system tend to be intangible, whereas the problems of the new system are all too tangible.