A large idea in modern poker theory is playing the Nash equilibrium strategy. A Nash equilibrium is where no player can improve their expected result by unilate

A large idea in modern poker theory is playing the Nash equilibrium strategy. A Nash equilibrium is where no player can improve their expected result by unilaterally changing their strategy, and two player games provably have the property that if you play any Nash equilibrium strategy, you expected value will always be greater than or equal to your EV when your opponent also plays an equilibrium strategy. The application of this is that at any given point during a hand you and your opponent have a "range" of hands (the term in the technical solver literature is an information set) that it would make sense for you to have from your opponents perspective. A range is a probalistic weighting of hands. Every action changes a players range, and for example if a player bluffs too much you can exploit that by calling with a bigger range against them. So poker becomes a game of, rather than figuring out what your opponent has, instead establishing a baseline strategy that can't be exploited no matter what your opponent does and then adjusting that to optimally exploit your opponent. That's the basic framework. Another book recommendations is "Modern Poker Theory".