For fiction: Waterstone's on Gower and Torrington and at Picadilly -- both have great literature sections. Note the store at Picadilly can be chilly so wear a sweater. Foyles is great too. Take some books and head to the top floor to look through them over a coffee and pastry. They have slightly different stock. Also, not yet mentioned is London Review Bookshop on Blury Place right across from The British Museum -- they too often have some odder/interesting works of fiction; small cafe inside.