eLISA/NGO will be a milestone of our understanding of the formation and evolution along the cosmic history of massive black holes residing in the center of galaxies. eLISA/NGO will explore almost all the mass-redshift parameter space relevant for reconstructing their cosmic build-up. The gravitational wave signal from coalescing black holes reveals their spin and redshifted mass, and the collective distribution of masses and spins of detected binaries will provide invaluable hints for discriminating among several possible formation and accretion scenarios. I will describe in details eLISA/NGO observations of massive black hole binaries, highlighting various aspects of signal modelling and parameter estimation, and discussing prospectives of constraining their cosmic history through gravitational wave astronomy.