Kyle Reese: Listen! And understand. That Terminator is out there! It can't be bargained with, can't be reasoned with! It doesn't feel pity or remorse or fear, and it absolutely will not stop. EVER! Until you are dead!
The Terminator: I'll be back.
Kyle Reese: Come with me if you want to live.
The Housekeeping: Hey buddy, you got a dead cat in there or what?
The Terminator: Fuck you, asshole.
Sarah Connor: Look. I am not stupid, you know. They cannot make things like that yet.
Kyle Reese: Not yet. Not for about forty years.
Kyle Reese: But, there was one man who taught us to fight, to storm the wires of the camps, to smash those metal mother fuckers into junk. He turned it around. He brought us back from the brink. His name was Connor. John Connor. Your son, Sarah. Your unborn son.
Kyle Reese: Nobody goes home. Nobody else comes through. It's just him and me.
Answer: The phone books are the reason for finding Sarah Conner. The Terminator didn't know what Sarah looked like or where she lived so he looked in there and when he saw three Sarah Conners listed, he was in Kyle's own words, "being systematic." In other words, going to each of the addresses listed for Sarah Conner and killing the women whose name were a match for his primary target. Kyle already knew what Sarah looked like as he was given a photo of her by Johnn Conner but, just like the Terminator, didn't know where she lived so he used a phone book to locate and protect her.