I read Start With Why because it's referenced by several others books I've read recently. It's good, but save yourself a lot of time and watch Simon Sinek's TED Talk on his Golden Circle idea. It covers the same material (sometimes nearly verbatim to the book), and it's more consumable.
The big takeaway, is that most people get their messaging wrong and try to make a rational argument about "what" they do and sometimes tell you how they do it. Sinek's makes the case that people care more about "why" you do something, and it makes for more compelling story telling. Further, a strong core "why" powers the company culture of some of the most dominant brands and companies in the world, which ties nicely with the idea of "Preserve the Core" from Built to Last.
Anyway... Sinek's recipe is to start with why you do something, explain how you go about your business, and the "what" is the proof that the first two items are true. Sinek claims that this structure, with the "Why" at the core, encircled by the "How", and then the "What", matches with structures in the brain, which is why it works so well.
Whether that's true or not, I can speak from personal experience and say that it's effective. Take Tulip for example...
"[SAP and everything else available ]for a modern operations worker is terrible, so we're building something better. We're empowering the front line worker. That's why we're here."
Fuck yes, I'm in. It almost doesn't matter what the product is, I'm here for the why, because I've been on the front line, and I want better tools.
The rest of the book is mostly details and anecdotes about what happens when a company does or doesn't have a core why. One take away that stuck in my mind is the use of the core why as a filter for what advice fits you and your business. Sinek uses a food analogy... "You should eat Oreos!"... "No, try wine every night!", "Eat well marbled steaks!", "Convenience food will save you so much time!" etc, etc... If you have a core why, like "We're here to help you throw the best parties", or "We're here to help you live a healthier life", or "We're here because we want to save you money", will help filter out which pieces of advice to follow, because they fit you.
Overall, I recommend this book, but I recommend the TED Talk more.