Two warnings up front: TIME (you can 'literally' spend hours here ; ) and EXCESS LUGGAGE (on your fight home).
Artwords, founded by the charming and extremely knowledgeable Ben Hillwood–Harris back in 2001, is one of my all-time favorite destinations in London – an incredibly well curated bookstore, that specializes in design, fashion, architecture, fine art, and photography. It's a bit like drinking out of a fire hose for me to spend an afternoon there, discovering new unusual books and magazines. Of course I always walk out with way too many books, although...I wonder if there's such a thing.
If you're exploring Shoreditch, it's a good idea to make Artwords your last stop. I've learned my lesson, lugging my treasures all over the place.