I've read two
poker books cover to cover: Dusty's Treat Your Poker like A Business and The Zen of Poker.
I'm big on the mental and discipline aspect of poker these days and I attribute my recent successes and vast improvements over the past 2 years to working on these two aspects.
It also doesn't hurt to brush up on things like BRM, tilt control,
odds, reading board texture, and c-betting understanding. Pretty basic stuff but I mean for me, once I combined understanding of just basic
pot odds, outs odds, and board texture's role in understanding odds with better discipline and mental focus my improvement was like day and night for me at least. I'm also no longer afraid to try different strategies on the felt with different ranges, positions, etc that might either go counter to what I've always done or counter to the popular opinion. Will this move work? Should it work? Does the math support it? Do these spots come up often? You should be actively asking yourself these questions as you play
hands. We all can build a great foundation of basics, but it's up to us individually to build upon it and really develop our game. Advanced books and training videos can help the foundation but I think actually hurt some players who stick to much to the strategy recommended in these advanced books and training guides. Talking over hands on forums like this and reading threads of hand analysis and such is a much better use of time I think.
I may consider reading some others BUT my issue with books and such is that it might get you set in certain ways/patterns/methods that could be less effective in today's game.
I think the best way to learn is on the felt. Grinding. Putting in time on the table. Yes look over hands, yes read and understand odds and bet sizing online for free. But books and training videos concern me because you're relying on someone to teach you their strategy which unless you then adapt to make your own and apply it on a hand by and basis you're just really another predictable player.
There's so much free info out there online, use it. Sure the books can be helpful, but so can applying basic concepts and then going beyond them as you experience thousands of hands each with their own different dynamics and challenges to solve.