Books are FREE because all Teachings have been given by Adi-Buddha or Shiva to the mankind to apply - hope the free stuff is well appreciated, because materialists believe that only that is good what costs a lot and when having in the own hands they are happy. The books substitute a lot of other books (growing like mushrooms in these days), which are just repetitions or new (or diffusing) interpretations of the essence of the Teachings  - save money! The number of books on buddhism and trantrism is uncountable (must be near 5.000 alone in english) - on yoga maybe more. Those 4 books are outstanding and for the practice only and not for dogmatics. In all 4 books there is a chapter "literature" with recommended books on fundamental philosophical knowledge ... but only practice itself proves everything!