She said, "Is this your book?" (Change into indirect speech).

She asked if that was her book.

She asked if that was my book.

She asked if this was her book.

She asked if this was his book.

In indirect speech, the pronoun "this" changes to "that" (because it refers to a specific book).

"Your" changes to "my" (because the pronoun "you" refers to the person being addressed, which is now the speaker).