The thing that helped us the most were Paul Graham's essays: http://www.paulgraham.com/articles.html

My personal recommendations are: http://www.paulgraham.com/boss.html http://www.paulgraham.com/13sentences.html http://www.paulgraham.com/startupmistakes.html http://www.paulgraham.com/ramenprofitable.html http://www.paulgraham.com/ideas.html

I think these essays were both necessary and sufficient for making our start-up successful (in terms of reading, lol).