I don't know what it says in the book, but I'd expect the message would be similar to the YC advice, which is to start talking very early and frequently to potential and existing customers, to ensure that what you’re building is highly valuable to them.
YC partners have often said that the companies that achieve the biggest success are the ones who are the most active in talking to existing/potential customers.
They also note that the ones who spend money on paid marketing before they have a highly valuable product and organic (i.e., free) growth are the ones who fail.