Beautiful Nicholas Hoare Books is another of those childhood favourites of mine you're probably tired of hearing about. I'll skip over my infant transports of joy and get to the point.
For literature, art books, and children's books, this is one of the best places in town. They stock wonderful things (including a good selection of Edward Gorey's little volumes) and have a bright, wood-floored store with comfy couches and a fireplace. I don't know their CD collections very well, but this is one of the best places I know to come for a gift - and then end up buying for yourself.
They also host literary events, are happy to help you find what you're looking for, and even have those bookcase ladders you see on British televison mysteries. I love them forever.