After doing the first-time CTO thing three years ago in an established company with over 100 engineers, I think these two are the minimum required reading:

An Elegant Puzzle: Systems of Eng Management (https://lethain.com/elegant-puzzle)

and

The Art of Leadership, small things done well (https://www.amazon.com/Art-Leadership-Small-Things-Done/dp/1...)

There are a lot more that were helpful to me, but those two encompass most of the important concepts and skills already in a usefully synthesized way, at least for me.