“When art critics get together they talk about Form and Structure and Meaning. When artists get together they talk about where you can buy cheap turpentine.”
As a founder, speaking to other startup founders is always special. After "make mistakes and learn from them", if I were to pick one other source of consistently accurate and useful advice it would be from fellow founders.
The advice a fellow founder can give you is qualitatively different from what get from books or experts who are not on the frontline, with a few exceptions. It does not stand on ceremony or discuss mere "strategy", and is centered around logistics. It is very direct, containing empirical evidence that may be counter-intuitive. It keeps in mind that you do not have unlimited time, resources, or knowledge.
Founders (or ex-founders) are able to give you the benefit of their learnings in a way nobody else can.