Elisabeth Haich: Sexuelle Kraft und Yoga.

This is a book you should definitely read (if your intuition tells you, it is ok - please let yourself not be deterred by the title, although the newest version is now even called "Yoga und Sex"). It is a book with very valuable and essential information for spiritual development as well as for a healthy and (very) long life.

It shows, how the basic life energy that flows within us all can be stored and used for one's spiritual development and to achieve things a lot of people might not have believed in more recent times.

Also see the other books by the author:

If you are interested in Qui Gong, the books by Mantak Chia might give you additional information.