what is the one thing you wish you knew before starting out ?I'd like to have been exposed to Steve Blank's works, The Four Steps To The Epiphany and The Startup Owner's Manual sooner. They are BTW, both the same book and not the same book. That is, TSOM is sort of the 2nd edition of TFSTTE, so in that sense they're the same book. But while there is a lot of overlap there's also plenty of material in each book that isn't in the other. So for anybody who is interested, I'd actually encourage you to read both.
Likewise, I would have liked to have been exposed to Jeffrey Thull's selling process, "Diagnostic Business Develoopment", which is described in his book Mastering The Complex Sale, Exceptional Selling and The Prime Solution. I'm a fan of his model and while I can't claim to have empirical proof that it works yet, it feels right somehow.
I would also like to have had the chance to read How To Measure Anything by Douglas Hubbard sooner. It's not a book that's about entrepreneurship, selling, or anything of that nature, but the ideas in the book strike me as broadly applicable to many domains. To give a tl/dr; it's roughly something like "use calibrated probability assessments to generate initial estimates, build a model possibly incorporating nth order effects, and then use a monte carlo simulation to build a probability distribution using the estimates and the model". There's a little bit more to it than that, but that's the gist (as I understand it anyway).
Finally, I'd say that I'd like to have been exposed to the ideas in *The Discipline of Market Leaders" earlier. The core idea in that book is that there are different bases for competition. One (obvious) one is price, and another pretty obvious one is "technical (product) superiority". But the book makes the point that there are other, less obvious ways to compete, like "operational excellence" (basically, running leaner and managing costs/defects, etc. better than the competition) and - my favorite - "customer intimacy". The last one basically means being more knowledgeable about your customer's business, making more tailored solutions, and basically becoming more of a partner than just a vendor. That happens to align well with the whole "Diagnostic Business Development" idea that Jeff Thull pitches, so I think these ideas complement each other well.
Outside of all that, my advice, FWIW, would be to say "you can't learn and understand enough about marketing and sales". Seriously, building technology comes easily to us... doing sales and marketing does not always do so. But I firmly believe that this stuff can be learned, and I think that if you're selling something you're actually passionate about (and if it's something you built, you probably will be) then you can learn to sell it without being a "natural born salesperson". Yeah, maybe some personality types take to selling more easily than others, but I think pretty much anybody can learn to sell to some degree... and in the early days of a startup, unless you're lucky enough to partner with somebody who is a "sales guy" type already, you probably will have to do the initial selling.