These two books are basically one and the same, as the Bruce Main-Smith one is a faithful reprint of the Temple Press one, to which only a couple of pages with the Norton frame/engine numbering system have been added, which can also be found in many another Norton book, so are not something one needs to buy this particular book for.

The Temple Press Book, first published in 1959, might not be available from your local W.H.Smiths anymore, and certainly not for 3S 6D; so the BMS reprint comes in handy.
The books are the best Norton had to offer in ways of technical information at the time regarding the single cylinder models built between 1928-55. Compared to modern workshop manuals they are not comprehensive, but the main aspects of the motorcycles are covered, and there are some very usefull datasheets on carburation, gearing and general engine data included. Also in the book are technical drawings, taken from parts lists and suchlike, so if you own a Norton from that era I should think you want one of these books.