What is the ruling on the prayer of one who places a book to prostrate on it, if the blanket he is praying on is soft and his face would sink into it if he were to prostrate on it?

The place of prostration must be stable, and prostrating on a book is sufficient. However, if the book contains the Qur'an, it is not permissible to prostrate on it due to disrespect. If it contains religious knowledge but not the Qur'an, prostrating on it is disliked. In all cases, the prayer remains valid.