I'd recommend Ed Miller's book, Poker's 1%. Most of his stuff is geared toward live games, but that particular book looks deeply at the math of winning poker.

Just a quick example, if your opponent bets half pot, he's automatically making money, and you're automatically losing money if you fold more than 1/3 of the time. So you need to continue with at least 2/3 of your hands, in order to prevent him from making money by simply auto-betting.

So even though you only have two cards at any given time, you need to know what your entire range is in that situation, and then you need to determine if your current holding belongs in the 2/3 of hands that continue, or 1/3 of hands that fold.

From there, you can evaluate every hand in the range and see what the score is. If you find you'd fold more than 1/3 of your range to that half pot bet, then your range isn't good. It means you made an error on the previous street.