Liberty opened in 1875, selling ornaments, fabric, and objets d'art from Japan and the East and is now a beautiful store expanding over many levels. Liberty is what Harrods wishes it were--unquestionably classy and unique. Each department is beautifully set out over wooden floors, but it's the book section which really holds my interest.
"Liberty celebrates originality and embraces a spirit of creativity," so it's safe to say that the quality of the books reflects this. Liberty sells a wide range of eclectic and high-market art books (when I was last there, I saw a glossy art book on shoes and Tibeten monks). You enter the book room from the large open-level main gallery, following the dark wooden steps.
If your budget can't stretch to the higher-end books, off to the sides of the book section are some very cute little journals and address books I've seen in the pages of glossy magazines. They make great gifts.
This is unquestionably the nicest place to browse for books in London.