I've been looking for a good introductory book on leadership. It's Your Ship by Capt. Michael D. Abrashoff is the best I've come across. If someone is a first-time manager, there are some great lessons in here.
The first challenge I see new managers and startup CEOs face is the transition from being an individual contributor to manager. It is hard. The most difficult thing is for these newly promoted managers having to treat their former peers differently. You can be friends with peers in a company. When you're their manager, you can only be friendly.
This is one thing the US Military does well. When someone is promoted, it usually comes with a transfer to a new post. A newly elevated manager gets to start fresh without the baggage of being advanced in front of their peers.
I recommend new managers find a mentor who demonstrates substantial managerial and leadership skills (they're not the same thing) to navigate this process. It can make a significant impact.