Insertion of commas around title

Could someone please help? Thanks.

Could someone reply to my question? Thanks.

My sister has read a book, The Old Man and the Sea, by Ernest Hemingway.

In that example, The Old Man and the Sea has, effectively, been inserted into the sentence: "My sister has read a book by Ernest Hemingway."

My sister has read the book The Old Man and the Sea by Ernest Hemingway.

In this example, the definite article requires that the title be inserted because Mr. Hemingway has written more than one book. Therefore, no commas should be used.

Does this make sense to you?

Thanks, Setanta.

Your explanation is simple and clear. I do appreciate it very much.

You're welcome, Miss Tan.