Question about the end. Spoilers

He's thinking how easily they could fall in love and be together, but then realizing unless she joins his life, or he joins the Amish life, it's not going to happen. He pauses just long enough to decide that Daniel is a very decent man and would be good for her, and continues driving.

Rest in peace, Roger Ebert. You were the best.

Maybe it's just to say goodbye. Maybe he tips his hat to him as if to say "she's all yours".

It's just the movie's way of telling the viewer that Book is acknowledging Daniel's farewell gesture (i.e. the tip of the hat). If the car has simply kept going, then the audience might have assumed that Book ignored Daniel. He probably said some nice words to Daniel, or he gave a wave of the hand (he did something).

While the scene as edited lends itself to tasteful interpretations of mutual respect, in the original take Harrison Ford leaned out his car window making obscene gestures and yelling, "Put a bunch of babies in her, pal! Name one after me!" He then proceeded to drive for several miles as the end credits played, continuing to make elaborate sexual gestures to no one in particular. In some ways, this original ending offered a larger emotional payoff to the film's story.

He stops to acknowledge Daniel before leaving.