Quite apart from having six floors of books (you can happily spend hours browsing) this store has a handy cafe and bar on the fifth floor which I found nice and relaxing having dodged into the store to get out of the rain!
www.waterstones.com/waterstonesweb/navigate.do?pPageID=200003
203-206 Piccadilly, London, W1J 9LE
+44(0)843 290 8549
Google map: bit.ly/W4Nfj8