Best books/guides for newsletter growth?

As some of you know, I'm writing my personal growth newsletter, Grit List, for the next seven ten years (here's why 10 years).

Now it's time to grow. I'm looking for articles/blogposts/books/guides/podcasts/videos–ANYTHING that really helps go into detail about building a newsletter.

After a lot of googling I have some good ideas and fundamentals, but am looking for more suggestions. While the guides I've read are good, they aren't that in depth. They generally map out "7 tactics to grow your list", breeze through the process, and that's about it.

Fundamentals that I've landed on are:

• write good content
• write content consistently
• promote on forums, twitter, reddit continuously (but don't spam)

Any recommendations for some long form content that goes in depth into growing a newsletter's audience?

Shout out to @yaroslawbagriy's awesome Newsletter Crew podcast (thanks for the great content, I just binged all 3 episodes!) and @AndrewKamphey's great post on profitable newsletters. You both have helped me a lot with getting started!