Parashara Hora
Parashara Hora is the most important book written on Jyotish by Maharishis Parashara. Its called the most important book because almost majority of Jyotishi in India follow the different systems described in it traditionally generation after generation.
http://rohanialoombooks.files.wordpress.com/2010/09/brihat-parasara-hora-sastra.pdf
Saravali