Alan Dean Foster wrote his books about Flinx and the Human/Thranx civilisations (including my favourites The Tar Aiym Krang, Orphan Star, The End Of the Matter, and Bloodhype) - many years ago, but as an imaginitive sci-fi series for me they're pretty much unsurpassed. In the Flinx series, a personal quest for an individual's origins becomes entwined with events of interstellar import.
Foster creates a populated universe rich in colour, religion, politics and intrigue indulged by wonderfully innovative life-forms. These stories are full of twisting plots and dark corners both of the universe and of the mind. Sinister churches and greedy corporations, busling alien markets and the deep emptiness of infinite space, all combine into a mind-bending tapestry of intergalactic life.