You need to describe the library model. There are 3 entities: "Author", "Book", "Reader". Physically, there is only one book and it can only belong to one reader. You need to create tables for the library to account for this.

To describe a library model with entities "Author", "Book", and "Reader" and considering that each physical book is unique and can be with only one reader at a time, the following table structure can be proposed:

1. Author

• id (PK)
• name
• other data (e.g., biography)

2. Book

• id (PK)
• title
• author_id (FK to Author.id)
• reader_id (FK to Reader.id, nullable)

3. Reader

• id (PK)
• name
• other data

Explanation:

• Each book is linked to one author via a foreign key.
• The reader_id field in the books table indicates which reader currently has the book. If the book is available, this field will be NULL.

This approach allows:

• Tracking who is the author of the book.
• Tracking which reader has the book (only one at a time, as the field is singular).

Example SQL table creation:

CREATE TABLE Author (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE Reader (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE Book (
  id SERIAL PRIMARY KEY,
  title VARCHAR(255) NOT NULL,
  author_id INT NOT NULL REFERENCES Author(id),
  reader_id INT REFERENCES Reader(id) -- NULL if the book is available
);

This design accounts for the fact that a book is physically one and can be with only one reader at a time.