Really hard to answer, as it all comes down on where you are at your PLO journey.
For basics, I think one of the best content for few bucks is Jnandez's book. Easy to get as e-book, doesn't cost a lot.
Once you know the basic stuff and go further, there are multiple roads to go:
- Mastermind is an option once you are familiar (and implemented to your game) with range thinking and the concept of gto (what it is, why we should be interested about it, should we try to use it). Quite expensive, my advice is to get it for a month or two and just watch videos and make notes. Less playing, more studying.
- RIO has some good stuff for Essential (the lower membership), I liked Emty's series.
- Forums are old relics and they are slowly dying. At the moment I think we have one of the best hand analyzing forums for small stakes PLO right here. It's somewhat active, and you get quite quality posts. Compared to other forums where most of the replies are off topic of toxic.
- Coaching/Study groups are one option. Of couse coaching costs and study groups are free. But if you find a coach that has what you need, for a price that you are happy to pay, it definitely is a good option to take the next step. Study groups are more of people in the same situation trying to go forward together. One option to get a coach for the study group, not sure though how many coaches are willing to do that.
What comes to Chambers book, I don't think anything is worth 2,5K (book 1 for fundamentals and book 2 for 6max PLO). The information is good, I have checked the free previews and it's a lot more than just GTO. It's about how to play versus different opponents on different situations and so on. Good stuff, but nothing ground breaking. Funny thing is that I have told years that at small stakes HU pots oop you can cbet quite wide with air versus most opponents. Average fold to cbet (at least was few years back on most sites) is often around 40-50% even in looser games. For a 3/4 cbet we need just above 40% fold equity to make immediate profit. So with air, often the best option is just to cbet bluff instead of check-fold. That is against solver strategy, and I received a lot of critisism for my approach. Some people said that solver doesn't do that, so neither should we. Now Chambers says the same thing in his book, on certain boards the best option oop HU with air is to cbet as the FE is there for immediate profit.
The biggest thing in learning PLO (or any other poker format) is to think a lot yourself. You can't memorize everything, you need to understand what we are trying, why we are trying and how we are doing it. Instead of just looking for answers, ask the questions you can't answer. And try to answer them. Learn to make mathematical models of situations and calculate them out. The more you do the numbers, the more you understand them. And poker is all about numbers and math.
Example, paired boards. Should we cbet them with high frequency? If you are just looking for the answer yes/no, it doesn't take you far. But if you explore the situation and start to understand what good/bad happens with high frequency, when it's +EV and when -EV, it gives you a lot more skills to play paired boards. It's like in science, we prove something by not being able to prove it the other way. Let's say the hypothesis is that we can cbet paired boards 100% with air and it's profitable.
To make it mathematical model, we explore what is average opponent's estimated calling range on different paired boards. Like 772r, JJTss, AA5r and so on. Then we can calculate how often their range hits the board, and maybe for couple of different preflop calling ranges. Then we estimate how often people bluff those boards. This is when Trackers come handy, we can see the raise cbet stat for population. Compare that to valueraise range frequency, and we see that average opponent doesn't really raise as bluff. For half a pot cbet we need 33% FE. If opponent folds more than that, it's immediate profit. Now the numbers give you the answer, it's either +EV or -EV with your assumptions and estimations.
I did that years ago, and my conclusion back then was that it's +EV.
Now the next question is if it's max EV line to cbet with high frequency? And how to adjust if opponents start to bluff raise, float or just call with wider range? And so the journey continues, and during it our understanding of paired boards and what affects our decision making improves. Instead of just wanting to know if we should cbet them often or not. Cos the answer to all PLO questions is: it depends.
I have sometimes done with my students a game analyzis to see where the student is and what needs to be improved the most. Just a paper/excel with different game areas and concepts/situations from preflop to river and student's own estimation of his skills and comfortable level in those. It can be from 1 to 3 (1 very unsure, 3 very confident), A to C, 1 to 5, what ever works. After that it's easier to make a study plan. And often the best way is to focus most on the most common situation. That's why it all starts in preflop, as we have play it in every hand. Then comes flop when we are PFR and so on.
Oh, and in above when I say "you" I don't mean YOU JoePlotter, but anyone who might read it or is in the situation. I got a bit carried away from the original questions. ♦