On the origin of Reiryoku

On the origin of Reiryoku is a book written by Kisuke Urahara about the way Reiryoku flows and  affects the environment around it.

Practical Uses
The book details a number of uses, some deadly, that Reiryoku can be used for:
 * Heaven's bodily reconstruction(天国の身体復興, Tengoku no karada fukkō):This is one of the most dangerous uses of Reiryoku. It is used in the form of a pill that stores pure Reiryoku that emits it around the body when dissolved, it has so much energy that it gives the consumer power  like that of a Shinigami and gives them the ability to wield a zanpakuto.
 * Studying this book allows you to develop new kido spells and bakudo and use low level spells without the need for any incantations.