Question: What's the meaning of 'IN my old paperbacks'?
The Rite Aid and the Shopwell are less than a mile from our house, in a little neighborhood strip mall which also supports a video store, a used-book store named Spread It Around (they do a very brisk business in my old paperbacks), a Radio Shack, and a Fast Foto. It's on Up-Mile Hill, at the intersection of Witcham and Jackson.

Question: What's the meaning of 'IN my old paperbacks'?

Question

Question: What's the meaning of 'IN my old paperbacks'?
The Rite Aid and the Shopwell are less than a mile from our house, in a little neighborhood strip mall which also supports a video store, a used-book store named Spread It Around (they do a very brisk business in my old paperbacks), a Radio Shack, and a Fast Foto. It's on Up-Mile Hill, at the intersection of Witcham and Jackson.

Question: What's the meaning of 'IN my old paperbacks'?

Answer

To do business in something basically means to trade, either sell or buy.  
Think of it as a phrase: business ( Noun) and in (Preposition)

What business are you in?
My business is in buying and selling old paperbacks.

Answer

I agree with Roy, but I would also add to his answer.  This sounds like the character is a writer and a published author. The used-book store apparently buys, sells, and trades many of his old books.

Answer

I also agree with the other two, but in addition to Noah's suggestion, the writer could be an avid reader who gives or sells his books to the second hand shop when he has read them.

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