One of my favourites is in Brewer Street, off Wardour Street in Soho (green paintwork, adult section downstairs). Also I've been recommended to Collinge & Clark, 13 Leigh Street, WC1, but I haven't been there yet.
Tim

Collinge & Clark is splendid if you are interested in private press books and more scholarly works on typography. If you are looking for graphic design books, then the design bookshop on Charing Cross Road (east side, block-and-a-half south of Cambridge Circus) is a better bet.

For contemporary graphic design
Magma Books has two locations:
http://www.magmabooks.com/

Waterstones in picadilly circus has a small but useful typography section.

The V&A museum contains the National Art Library which allows you to photocopy (at a premium cost).
http://www.vam.ac.uk/nal/index.html

Whether you're looking for a certain issue of graphis from 1944 or a victorian lettering catalouge. The NAL is better than any bookshop.
online catalogue:
http://ipac.nal.vam.ac.uk

Faulkner Fine Papers 74 Southampton Row, Bloomsbury, WC1 (020) 7831 1151 (nearest station is Holburn on the Picadilly line)has a nice selection of books On Type, Bookbinding, calligraphy etc)

Another favourite stop, for books on printing history, palaeography, calligraphy, etc. is the British Library bookshop. This is a short walk north of Collinge & Clark.

Great, thank you all for the quick and useful answers, they helped a lot

Maija

You should also pop into Zwemmers on Charing Cross Road (is this what you meant, John?), and Foyles, a larger more general purpose but independent bookshop a bit further up the same road.

Tim, if its the same one, the shop you mention is called Soho Books, and was the very first place I saw my own book on display

Yes, I meant the Zwemmer's design shop, if that is indeed what it still is. Ownership seems to have changed at least twice in the past few years, and I couldn't remember if it was Zwemmers now or nor.

Thanks David, I tend to navigate and give directions by pubs or colours, which is not to everyone's liking but I know where I am, some of the time. Was your book upstairs or downstairs:?