Best "Advanced DSP" book
Assuming you have completed Oppenheim's Discrete-Time Signal Processing book then the next (advanced) step could be anyone of the followings: (assumes a graduate level Linear System Theory background)


Statistical Digital Signal Processing, Monson Hayes
Multiresolution Signal Decomposition: Transforms, Subbands,Wavelets, A.Akansu 
Adaptive Filter Theory, Simon Haykin
Multirate Digital Signal Processing, Rabiner
Discrete Random Signals and Statistical Signal Processing, C.Therrien
Fundamentals of Statistical Signal Processing : Estimation Theory, S.M.Kay
Multirate Systems and Filter Banks, P.P.Vaidyanathan
Wavelets and Filter Banks, Gilbert Strang
Multidimensional Digital Signal Processing, Dudgeon


To name just a few...