The Marks & Co bookshop at 84 Charing Cross Road, London, closed in 1970 - but the building still stands, and on the wall is a plaque commemorating Helene's book about her correspondence with the bookshop:

Text on plaque: 84 CHARING CROSS ROAD THE BOOKSELLERS MARKS & CO WERE ON THIS SITE WHICH BECAME WORLD RENOWNED THROUGH THE BOOK BY HELENE HANFF

The building's present use is as the Covent Garden Record store...

The building itself is referred to in several web pages - links follow:

MORE STUFF TO BE ADDED SHORTLY!!!!!!!