[lettergram]

I also highly recommend Startup School, also provided by YC:

https://www.startupschool.org/

That's what helped me get my most recent project ready:

https://projectpiglet.com/

I'd describe it almost as a "startups anonymous" group, where you have a mentor who's went through a startup, weekly talks, and a group to help you get off the ground.

In regards to the resource you've read, I can only tell you what I have seen. To some extent, the viability of your business comes from the team and future vision. You have to vet with customers, know the space, be excited etc.