I found Reflections: The Piano Music of Maurice Ravel by Roberts an interesting read that gave me much interesting information on the composer and his works.If you analyse his pieces harmonically, you can see that he has firm roots in basic harmonic functions, on top of which he branches out with more interesting colorings, mediants, and modal harmonies than his predecessors. He has some things in common with Debussy in this regard, while at the same time being radically different, and dare I say more advanced and refined than Debussy in this sense, as he anchors much more of his harmony in tradition than Debussy.