By contrast with chess you can learn every single rule in about 5 minutes, and then spend the remaining 80 years of your life exploring their depths.
Complex != deep, simple != shallow.
That might be true for the game of Go with its much simpler rules, but for Chess I reckon it takes at least an hour for someone to absorb all rules.
Ideally the learning process can be both fun and high-utility, through minigames and the other activities laid out in the book! It took me 3+ weeks of multiple hour-long classes each week to get a classroom of smart 4th graders to absorb the knight movements and play Knight Battleship competently. In the end, they did enjoy it... and once they felt mastery, they wanted to learn more.
I really love chess, gotten into it back in 2018 and learned a lot. Since mid 2020 I got into the bad habit of just playing 1+0 bullet games mindlessly without caring about the result just to shake off the boredom. Always repeating the same old openings and attempting the same old boring traps.
I wish I could get back to that "never gets old plus ultimate replay-ability factor", maybe I should start taking on 15+15 games back again.
Correspondence games will help you to understand chess more deeply, fix your own mistakes and to become an overall better player. You can also open many correspondence games in parallel, this way you don't have to wait for days until you can make another move.
You can never be 100% certain, but there's a few things one can do. Before playing an opponent in correspondence check his profile. Never play against new accounts (e.g. join date in profile < 3 months). Don't play someone who never lose games (e.g. 80 wins, 20 draws, 0 losses is suspicious unless it's Magnus). Don't play someone who only has very few games played in total. Compare the other statistics if he has a beginner level blitz rating of 1600 and rapid rating of 1700, but correspondence is GM like level of 2500+ then this is also suspicious.
15+15 sounds great but you can also go for 30 or 45 or 60 minute games online! Even if not in person, with those time controls at the low end of classical, you will find yourself thinking more deeply to re-wire bad habits. The mistakes will have more investment behind them, so you will tend to review the games and learn/internalize from them more. Especially if you find a good sparring partner, or set the rating range -0 +500, and only play people who are better than you. I'm on Lichess if you like: https://lichess.org/@/andytratt :)
I disagree a bit with correspondence because I think few people are wired to take it seriously. I've been doing correspondence Go and some days I just make a move because I'm busy with other things, so it really has to be a priority over a long time period. Also, I tried Diplomacy a couple times last year with various groups (https://www.backstabbr.com/) and found that it's a rare breed of human who, even in hacker demographics, can focus on such correspondence activities to my (very minimal) level.
It always struck me as strange, among the many chess shelves in the Mechanics Institute and elsewhere, that there wasn't such a starter book out there already...I'm very pleased to hear it stacks up well against your hundreds!
P.S. I have another one for parents teaching kids, which is basically a simplified and expanded chapter 1 of the adult one: https://andytrattner.com/img/pdf/teach-chess.pdf