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.