Unusual Americana . . . if you really want to read about it; American Beat, by Bob Greene. New York: Atheneum. 301 pp. $15.95.

Think of the guy who sits right next to you on a bus. Especially when he could have chosen an empty seat elsewhere. Someone who has a lot of interesting stories to tell - about where he's been, what he's done, and how unusual all the people are that he's met. The kind of person who can make an otherwise boring trip, well, interesting.

Bob Greene is always trying to be that guy.

He tells his stories not on the Greyhound but in his syndicated newspaper and Esquire magazine column. ''American Beat'' is a collection of his chosen best. Here are stories about Richard Nixon and Hugh Hefner, teenage beauty queens, scared children in Atlanta, heartbroken lovers, men who never learn there is strength in sharing emotions, and many, many others.

But Greene is far too free with people's privacy for me. An easy talker, he makes people feel comfortable with him. Maybe they shouldn't be.

I really didn't want to sit, listen, and watch an abortion. Or chat with Richard Speck, the mass murderer of eight student nurses in Chicago. Why does Greene think his reader wants to attend a lingerie party? Would reporters rush to feel bullet holes in the chair at the Utah prison where convicted murderer Gary Gilmore was executed by firing squad?

I'm sure the stories come across better when read one at a time, with the interval of a day or two between them. But because the stories are collected between the covers of a single book, we don't read them the way we read a paper over breakfast.

For the literary critic who would ask if there is an informing vision in Bob Greene's stories, I would have to identify two:

* Most of his stories are delivered with moral neutrality - as though Greene were saying: ''Don't judge, just tell. Detail the details.''

* People enjoy reading, Greene also seems to be saying, an honest attempt to tell about unusual people.

There are risks, however, in letting the guy who sits next to you on the bus lead the conversation. What's significant to him just may not be to you. And his presence imposes itself on the quiet of looking out the window alone with your thoughts.

But as a reporter, I make a deep bow to Bob Greene as a man who can write the simple declarative sentence, the sine qua non of readable journalism. He's mastered the craft of clarity.