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Today, my daughter and I had this brief conversation.

Daughter: "I finished reading the book."
Me: "Lestat."
Daughter" "No, Pigman."

Lestat and Pigman are names of books. She had to read Pigman for school while she had checked out Lestat from the library on my library card.

She said the book because she knew which book and assumed I knew which book. Assuming we had the same book in mind, the would be the correct article. She and I found out, however, that I didn't know which book she was referring to.

The is used when both the speaker and the listener know which specific thing or person is being referred to. The same goes for writer and reader.

What my daughter would have said, if she knew I did not know which book she was talking about is: "I just finished reading a book." Or "I just finished reading Pigman."