I will not indicate a book, but a very important tip: After my thousands of hundreds of
hands that I played heads up, I came up with a pretty basic structure of how HU matches should be played in the beginning before any specific adjustment for the opponent. To begin with, you should open 100% of your hands on the button, yes up to 27o. That is because of two reasons: # 1 we are going to make an immediate profit if our opponent plays very tight out of position and # 2 we want to force him to Throwing many hands out of position against us soon we can use our positional advantage to overpower it. Open 100% of the hands will also lead to tilt many people to do stupid things, more often drastic adjustments against us. So the structure for pre-flop position is to open every hand on the button up to 3x the blinds.