Advanced astrology books
Here are some advanced astrology books that I'd recommend.
Predictive
Identifying Planetary Triggers and Predicting Events with Astrology by Celeste Teal. My favorite author. She also has this website:
http://www.moonvalleyastrologer.com/
Predictive Techniques Regarding Romance by Ana Ruiz
When Planets Promise Love by Rose Murray
Solar Returns
The New Solar Return by Merriman.
Relationships:
Astrology: A language of Life: Volume IV - Relationship Analysis by Robert P. Blaschke
Love Formulas - 2 by Nance McCullough
The Only Way to Learn About Relationships: Volume 5: Synastry Techniques. by Marion D. March and Joan McEvers. Actually I'd recommend the whole series.
Other
The Rulership Book by Rex E. Bills
These are the books I use over and over in my professional practice and research projects. Some of them are hard to find or out of print, but Amazon is probably your best bet.
I'd also recommend
http://www.cafeastrology.com/
as a very good site for both beginners, intermediate and a little advance too.