While I haven't read it (it came out after I'd left the "game"), what *Four Step...

While I haven't read it (it came out after I'd left the "game"), what Four Steps.... covers is absolutely critical, I've seen and been in too many startups that failed due to failing in Customer Discovery in one way or another.

In earlier times, here are the books I found to be particularly useful: http://news.ycombinator.com/item?id=1021570

Others have endorsed E-Myth and I'll note that what's most valuable in Walking the High-Tech High Wire is in part their Customer Discovery story....