Where to begin? - Multi source polyphonic pitch detection I'm quite interested in real time pitch tracking. I have used mcleod pitch method, Yin and other methods to perform real time pitch tracking for a monophonic audio track. 

I know that Polyphonic pitch detection is at a whole different level and I want to know where to start. If the experts(masters) could point me at the right direction, It would be greatly appreciated. 
Papers, books, Topics or any suggestions that you could give me would be of great help.
It's a research topic. So try the music-ir.org MIREX web site for a vast number of academic papers and transcription competition entries. 

If you want to try a newer direction, try gathering many 100's of hours of snippets of tagged music, fuzz it, and feeding that as training data to a DNN that includes convolutional+LSTM layers.