Happy birthday Mr Debussy. Schirmer Performance Editions have brought out a selection of pieces suitable for late intermediate to early advanced pianists/ grade 5 + level: http://www.halleonard.com/product/viewproduct.do?itemid=296711&lid=0&keywords=debussy+harding&searchcategory=00&subsiteid=64
Other intermediate pieces by Debussy include: "Le Petit Negre"- this is written in the style of a cake-walk dance and has irresistible rhythms in the outer sections with a contrasting more lyrical section. It is an impressive and fun piece; "The Girl with the Flaxen Hair"- provides an introduction to Debussy's more mature style(described as Impressionist or Symbolist) and his other Preludes and is approx grade 5 + level.
After you have tackled these pieces and been lured into the marvelous piano sound world of Debussy you might like to explore: "Suite Bergamasque" (Prelude is on ABRSM's 2013-14 grade 8 piano syllabus), "Estampes" and "Preludes" books one and two.
I have found that Marguerite Long's "At the Piano with Debussy" offers interesting insights into Debussy's piano music and Roy Howat's " The Art of French Music" is a fascinating and essential book for anyone interested in French piano music and has a very erudite and useful chapter entitled Debussy and the Orient where he discusses Debussy's "harmonic chemistry". Fabulous stuff!