At the beginning of Back to the Future, it is clear that in 1985 Marty McFly knows where Doc Brown lives.
Why then did he look up Doc Brown's address in a 1955 phone book? Doesn't he already know where the Doc lives?
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In the beginning of the movie, when we see all the clocks, there is a shot of framed newspaper clippings (the mansion burned down on August 1, 1962). Marty has probably read this at some point.
Doc's address in 1955 is shown as 1640 Riverside Drive:
Doc's house we see in the beginning of the movie, in 1985, seems to be the garage of the mansion:
Notice the number on the fence:
It seems like in 1985, Doc is living in the garage of his former mansion, with the house number "1646", instead of "1640".
Also, by 1985, Riverside Drive has been renamed John F. Kennedy Drive (this is mentioned in the scene where Marty has Dinner with the Baines family), which is why Marty doesn't recognize that it's basically the same address.
He knows where Doc Brown lives in 1985. There's no guarantee that he lived in the same house 30 years ago. Indeed, it seems that he didn't, because once Marty finds his address, he has to (try and) ask for directions to it.
Marty: Do you know where 1640 Riverside--
Lou: Are you gonna order something, kid?
He wouldn't need to ask where 1640 Riverside was if that was still Doc's address in 1985.
Adding to the previous answer which is essentially correct that Marty didn't knew the previous address. It may be because of following reason:
The 1985 Doc Brown lived in a house which was way smaller than the mansion which Doc has supposedly owned back in 1955.
Maybe Marty knew that the mansion was burnt down and Doc who was once wealthy went broke.
Although initially wealthy because of his inheritance, he spent his entire family fortune on his time travel project. When the Brown mansion was destroyed by fire in 1962 and the property sold to developers, Doc subsequently resided in the mansion's garage. Once broke, he established a privately owned business to offer 24-hour scientific services, building ingenious devices for his customers.
But back in 1955 he owned a mansion and hence Marty sought phone book for Doc's address.